i believe these 2 are in conflict:
B: 1 mboxer = 1 player - correct
F: 1 player = 1 character - correct

the implication would be that 1 mboxer == 1 character

i see where you are trying to go with the logical proof. let me try:
(note: mb== multi-boxer sb == single boxer)

A: 1 mb == 5 characters
B: 1 sb == 1 character
C: 5 characters == 5 characters
D: 1 mb != 1 sb

but this is obvious since the power wielded by a mb is vastly superior to a sb

but, as far as the game world is concerned...

E: 1 mb == 5 characters
F: 5 sb == 5 characters
G: 1 mb == 5 sb

i see this conclusion to be inaccurate as it does not factor in the response times and strategy differences.
so i modified it:
(note: rt == response times + strategy difference)

E: 1 mb = 5 characters + 1 rt
F: 5 sb == 5 characters + 5 rt
G: 1 mb != 5 sb

so... you end up with a multi-boxer is not the same as 5 individual players.
but, since 1 mb == 5 characters + 1 rt and 1 sb == 1 character + 1 rt ... you end up with:

H: 1 mb > 1 sb

which is obvious as well. now we have proof.

Multi-boxers Rock!