Sorry; I cannot help you directly.
My rule of thumb is that if formulas are getting too complex to
understand/recall clearly, it's time to use a proper programming
language instead. The nice thing about org is you can have tables as
inputs to and outputs of src blocks...
My papers often have awk source blocks that process tables to generate
statistics for some (numerical) experiments.
Spreadsheets, org tables being an example of such, are brilliant tools
for simple calculations but are pretty much "write only programming
I had never heard of using an org table as input into a source block. That's really interesting.
The OP also poses an interesting idea that I have occasionally wanted. I have generally accomplished this by simply including the documentation above or below the table. In other words just having human text around it that says 'This =formula= bit is because of X'.
But the idea of a multiline TBLFM syntax seems to be already be almost supported. I was mildly shocked to find that this mostly works
| a | 1 |
| b | 3 |
| | 4 |
Note that I don't need the usual `::` separation between formulas. Where it breaks down is that I can't seem to reevaluate the whole table's formula by whacking `C-u C-c C-c` anymore and, obviously, there's no syntax for adding comments. Also I doubt that I'd be able to use any of org's keys for editing formulas. I wonder how hard it would be to extend org's understanding of the TBLFM to allow for this kind of syntax.
Once you had it then adding a comment character to it should be very simple.