3.13 Reexamined

[saracv3]

Aside from glu10's question about Assault and 3.13, I happened to run into the following situation tonight:
A Colonel (10-2-1) activates two mortars and scores a very nice DM result on two enemy pieces (one leader and one inf. unit, same hex). He then decides to activate a MG unit in his stack and a 75rcl weapon underneath a Captain in a an adjacent hex.

Due to that result, he then decides on the following:

He didn't use his combat modifier w/ the mortars (no advantage).  Can he use his modifier for for the  weapon? A Captain (9-1-0) also is located in the same hex as the Colonel, while a another Captain (8-0-0) is located w/ the weapon. Can the 9-1-0 Captain apply his modifier to the MG while 10-2-1 Colonel applies his modifier to the weapon? This would equate to +3 for the attack in combat modifiers and 13 for the MG and weapon. The 8-0-0 Captain becomes a non-factor.

But I think the problem lies in the sudden decision &  advantage of this 2nd attack, which may violate  rule 3.13.