Create IF Statement to find nearest date based on Unique Key (Power Query)

See the working here - Open a blank query - Home - Advanced Editor - Remove everything from there and paste the below code to test (later on when you use the query on your dataset, you will have to change the source appropriately. If you have columns other than these, then delete Changed type step and do a Changed type for complete table from UI again)

Challenge 1

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WCijKTylNLlEwVNJRMtQzMDAIDXYBMfUN9Y0MjECiRjBmrA66ciOYclOEckMDPOpNYcYb6ZvANRjpG5li6jCC6DCG6jBG2GCGzQKIchNTM4hyM4RyC9zK4e5HOBrMNjKG6ogFAA==", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [Product = _t, Price = _t, #"Valid From" = _t, #"Valid To" = _t]),
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"Product", type text}, {"Price", type text}, {"Valid From", type date}, {"Valid To", type date}}),
    #"Added Index" = Table.AddIndexColumn(#"Changed Type", "Index", 0, 1, Int64.Type),
    Custom = Table.ReplaceValue(#"Added Index",each [Valid To],each try if #"Added Index"[Valid From]{[Index]+1}<>Date.AddDays([Valid To],1) and #"Added Index"[Product]{[Index]+1}=[Product] then Date.AddDays(#"Added Index"[Valid From]{[Index]+1},-1) else [Valid To] otherwise [Valid To],Replacer.ReplaceValue,{"Valid To"}),
    #"Removed Columns" = Table.RemoveColumns(Custom,{"Index"})
in
    #"Removed Columns"

Challenge 2

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WCijKTylNLlEwVNJRMjUwCA12ATIM9Q31jQyMDJViddBUmEJVmOBWYQhRYYZFhRHIbAM8RkAUQE2wgCuIBQA=", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [Product = _t, Price = _t, #"Valid From" = _t]),
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"Product", type text}, {"Price", type text}, {"Valid From", type date}}),
    #"Added Index" = Table.AddIndexColumn(#"Changed Type", "Index", 0, 1, Int64.Type),
    Custom = Table.ReplaceValue(#"Added Index",each [Valid From],each try if #"Added Index"[Product]{[Index]+1}=[Product] then [a=#"Added Index"[Valid From]{[Index]+1},
b=List.Generate(()=>[x=[Valid From]], each [x]<a, each [x=Date.AddMonths([x],1)], each [x])][b] else {[Valid From]} otherwise {[Valid From]},Replacer.ReplaceValue,{"Valid From"}),
    #"Removed Columns" = Table.RemoveColumns(Custom,{"Index"}),
    #"Expanded Valid From" = Table.ExpandListColumn(#"Removed Columns", "Valid From")
in
    #"Expanded Valid From"