Multiple column date comparison and replace with earliest date

Hello,

I am a relatively new user to PowerBI. I am finding it challenging to implement some of the requirements using powerBI.  

I have 4 date columns, DateA, DateB, DateC and DateD. (Some of these may have blank value)  

I want to loop through these and do the following:

1. If any 3 of the 4 columns have the same date value, then compare that value with the 4th column to find the earliest date and replace all 4 dates with this lowest date. only non blank columns should be replaced. 

2. If out of the 4, some are blank, ignore that for the comparison. i.e if one column is blank, then if 2 columns are same, it can be compared against the 3rd one and earliest date found. 

3. if 3 of them do not match, then flag entire row as "manual intervention"  

Without having "for" loops, this is a bit challenging for me. 

I managed to identify the earliest date for #1 and "manual intervention" for #3 with the help of a bunch of if statements, but:

a. If any column is blank, I dont know how the logic will work

b. I dont know how to flag and replace all the cells with the lowest value found here. 

c. General question. Can "if - then" have multiple statements after then. like if a=b then { c=d; e=f} ? 

Here is my code to identify #1 and #3 above: 

= Table.AddColumn(#"Added col for FutureHire", "FutureHires-Updated all dates with original hiredate", each
if ([DateA] = [DateB]) then
     if ([DateB] =[DateC] ) then
           if [DateC] < [DateD] then [DateC] else [DateD]
     else if ( [DateB] = [DateD]) then
            if ([DateC] < [DateD]) then [DateC] else [DateD]
     else "this condition should not arise"
else if ([DateB] = [DateC]) then
        if ([DateC] = [DateD]) then
               if ([DateA] < [DateD]) then [DateA] else [DateD]
        else "this condition should not arise"
else if([DateA] = [DateC] and [DateA] = [DateD]) then
           if ([DateA] < [DateB]) then [DateA] else [DateB]
else "Manual Intervention" 

Is my approach completely wrong here? can you help guide me on how to achieve this? To acheive #2 above (if one column is blank), I need to have whole another set of if conditions? 

Thanks in advance,

Jas