if for the letter B of SEQ, the first LBL line says I, the second I, the third M and the fourth I, we will use I in the consensus label line.

I don't see a letter B in the SEQ line of your example, but I guess you mean the following (again using your example):

• The first letter in SEQ is M. All first letters in LBL are I, so you associate M with I.
• Further back, SEQ contains a W. The corresponding places in the LBL lines (same column) are M, O, M and M, so the majority would be M. We associate W to M.

It is not clear how to associate, say, the letter K, because the first K has only I in the LBL lines, while a later one has only M. also, what is the consensus if there is the same number of, say, I and O? Maybe you could specify your task in a more precise way?