Combat and Multiple Combat questions

[Umbar]

I have a question regarding the way combats are resolved in the assault phase. Here's the situation.

I have 4 plague marines (P) locked in combat with 1 Terminator(T) with a power fist. Board position looks like this after my assault phase ends. (tjhe position isn't perfect but it'll convey the idea.

_T
P P P P

On my opponents turn, he assaults My plague marines with a squad of tactical marines (M). Board looks like this after assault moves are done but before combat begins:

_T____MMM
P P P P MMM
_____MMMM

My opponent proceeds to resolve combat. He says that he can choose to resolve the tactical marines combat first. During that combat his terminator will not strike and any of my plague marines that want to attack the tactical squad can do so.

The way I understand the rules, this is a multiple combat, but it's still only 1 combat. Everything engaged with my plague marines goes in initiative order, directing attacks as described in the multiple combat section of the rulebook. So the tactical marines would go on init 4, attacking my plague marines. Plague marines would go on 3, directing attacks per multiple combat rules, and then the terminator would go initiative 1.

Here are the only rules I could find that were even slightly relevant:

There may be several separate assaults being fought simultaneously in different parts of the battlefield. If this is the case, the player who's turn it is can chose the order in which to resolve the combats, completing each combat before moving on to the next one, and so on until all combats are resolved.

Combats that involve more than two units are called 'multiple combats'

When determining assault results in a multiple combat, total up the number of wounds inflicted by each side to see which side is the winner. Every unit on the losing side has to check their Morale (they all use the same penalty, as described int he Morale section).... (stuff about sweeping advances) ... After assault resolution, all units that were involved in that multiple combat must make pile-in moves towards enemies that fought in that combat.

My only argument is that my opponents idea of how to resolve combat makes a lot of these rules I listed above make no sense.