Matching Data that could be in different columns

Great - I'm so glad we're making progress!  This line of the script determines which column number should be returned.

            else List.Last ( d{t} )  

 To get the column's position dynamically (instead of specifying the last column), add a step before add_dbAccount:

    column_dbPosition = List.PositionOf ( Table.ColumnNames ( dbData ), "Account ID" ),

And then reference it instead, like this:

else d{t}{column_dbPosition}  

Here is the complete new script with the column number dynamically referenced.

let
    // database data
    dbData = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("dY7BDoIwDIZfZdnZmKggcFs4ePCKN8JhaIUhY4bB+9uOgZBosnb9/q7/muf8aiy8a77DAjCnrakq65B0tnBjQGCUjvd3o1Gzo5XdpIhKS9V6/XA8BbzY5TyjByhk48bbyV/rfza/vlysb4pwyplWQz0Rm+tBaVGbYWXHMMIgStx4Cl0jtaLlsKTcg3xM5BpsFkovlMSb/bAjMDwFURw660sPNOeveR/CZbmnfAGQk4CVH54kjs68KD4=", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [#"First Name" = _t, Nickname = _t, Surname = _t, #"Full Name" = _t, #"email 1" = _t, #"email 2" = _t, #"Account ID" = _t]),
    dbData_ChangeTypes = Table.TransformColumnTypes(dbData,{{"First Name", type text}, {"Nickname", type text}, {"Surname", type text}, {"Full Name", type text}, {"email 1", type text}, {"email 2", type text}, {"Account ID", Int64.Type}}), 
    // transform the table to a list of lists.
    dbData_List = Table.ToRows ( dbData_ChangeTypes ),
    // form data
    formData = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("bY3BCoMwEER/ZclZ+g0hBwte7S14iHUaY90sxPT/K1sLFnqYy7zHjPdmEdg94yoxbpe7sGlMJyCnhRkab0Zku+eADplcQZiU1cR2lsohrQe/JaaeU52V/5vvX1vI54MIKRGLxWnmqh11kvGRHuEJFX60tmCi9ns4vAE=", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [Email = _t, Name = _t]),
    formData_ChangeTypes = Table.TransformColumnTypes(formData,{{"Email", type text}, {"Name", type text}}),
    // Extract delimited parts of the name, so they can be evaluated separately.
    // Take caution when using Text.Split as extra delimiters can throw off the results.
    // Ideally, the form fields would be similar in structure to the db to be matched, but that is not the case here.
    // Edit/add steps if additional delimited parts need to be accomodated, like name suffixes.
    formData_NamePart1 = Table.AddColumn(formData_ChangeTypes, "Name Part 1", each Text.BeforeDelimiter([Name], " "), type text),
    formData_NamePart2 = Table.AddColumn(formData_NamePart1, "Name Part 2", each Text.AfterDelimiter([Name], " ", {0, RelativePosition.FromEnd}), type text),
    // Add a list object to each row, containing each record's values for specific columns in the table.
    // If steps were added for additonal name parts, also add them to the list of fields (r) in this step.
    // Match the form data to the db data.
    column_dbPosition = List.PositionOf ( Table.ColumnNames ( dbData ), "Account ID" ),
    add_dbAccount = Table.AddColumn (
        formData_NamePart2, 
        "Account ID", 
        each 
        let                                              // Variables
            r = Record.ToList (                          // List of specific fields 
                Record.SelectFields ( 
                    _, { "Email", "Name Part 1", "Name Part 2" } 
                )
            ),                                           // Current record values as a list
            d = dbData_List,                             // db list of lists 
            t = List.PositionOf (                        // Position of match in the db
                List.Transform (                         // --> list of 
                    d, each List.ContainsAll (           
                        _,                               // db lists
                        r,                               // containing form data elements
                        Comparer.FromCulture (           // Apply current culture 
                            Culture.Current, true        // with case insensitivity
                        )
                    )
                ), true 
            ),
            i = if t = -1                                // = -1 if no matches were found
            then null                                     
            //else List.Last ( d{t} )     
            else d{t}{column_dbPosition}                 
        in 
            i,
        Int64.Type 
    ),
    remove_columns = Table.RemoveColumns (
        add_dbAccount,
        { "Name Part 1", "Name Part 2" }
    )
in
    remove_columns

Alternatively, you could use a function, like this: (I named the function fnGetDataFromExternalTable )

let
    fn = ( formsTable as table, dbTable as table, dbColumnName as text ) =>
let
    // database data
    dbData = dbTable,
    dbData_ChangeTypes = Table.TransformColumnTypes(dbData,{{"First Name", type text}, {"Nickname", type text}, {"Surname", type text}, {"Full Name", type text}, {"email 1", type text}, {"email 2", type text}, {"Account ID", Int64.Type}}), 
    // transform the table to a list of lists.
    dbData_List = Table.ToRows ( dbData_ChangeTypes ),
    // form data
    formData = formsTable,
    formData_ChangeTypes = Table.TransformColumnTypes(formData,{{"Email", type text}, {"Name", type text}}),
    // Extract delimited parts of the name, so they can be evaluated separately.
    // Take caution when using Text.Split as extra delimiters can throw off the results.
    // Ideally, the form fields would be similar in structure to the db to be matched, but that is not the case here.
    // Edit/add steps if additional delimited parts need to be accomodated, like name suffixes.
    formData_NamePart1 = Table.AddColumn(formData_ChangeTypes, "Name Part 1", each Text.BeforeDelimiter([Name], " "), type text),
    formData_NamePart2 = Table.AddColumn(formData_NamePart1, "Name Part 2", each Text.AfterDelimiter([Name], " ", {0, RelativePosition.FromEnd}), type text),
    // Add a list object to each row, containing each record's values for specific columns in the table.
    // If steps were added for additonal name parts, also add them to the list of fields (r) in this step.
    // Match the form data to the db data.
    column_dbPosition = List.PositionOf ( Table.ColumnNames ( dbData ), dbColumnName ),
    add_dbAccount = Table.AddColumn (
        formData_NamePart2, 
        dbColumnName, 
        each 
        let                                              // Variables
            r = Record.ToList (                          // List of specific fields 
                Record.SelectFields ( 
                    _, { "Email", "Name Part 1", "Name Part 2" } 
                )
            ),                                           // Current record values as a list
            d = dbData_List,                             // db list of lists 
            t = List.PositionOf (                        // Position of match in the db
                List.Transform (                         // --> list of 
                    d, each List.ContainsAll (           
                        _,                               // db lists
                        r,                               // containing form data elements
                        Comparer.FromCulture (           // Apply current culture 
                            Culture.Current, true        // with case insensitivity
                        )
                    )
                ), true 
            ),
            i = if t = -1                                // = -1 if no matches were found
            then null                                     
            //else List.Last ( d{t} )     
            else d{t}{column_dbPosition}                 
        in 
            i,
        Int64.Type 
    ),
    remove_columns = Table.RemoveColumns (
        add_dbAccount,
        { "Name Part 1", "Name Part 2" }
    )
in
    remove_columns
in 
    fn

Invoke the function:

fnGetDataFromExternalTable ( formsDataTable, dbDataTable, "Account ID" )