I don't think it's possible to have a calculation get a function name from
some other cell.. (?)

Indirect is how you would grab letters and numbers to be used in cell/range
references, but you probably already know that.

It might be easier if you tested what you were trying to do on a very small
range, one piece at a time. It would also make it easier to interpret what
you're trying to do for those reading this NG, if you explained it on that
scale. If you don't get anywhere with your current formula, try it on a very
very small scale and post it here and people will probably know very quickly
what you're trying to do.