End Game Combat Dispute

[Chupacabro]

Last night 3 of my friends were on a Munchkin bender at 2am when all hell broke loose. While playing standard Munchkin (2014 ed.) I was a level 9 and they were both level 8's. I had an effective fighting power of +22 and when I looked for trouble I played the level 4 Undead Horse. Since I clearly had the horse beat, they then wandered in a Level 18 Squidzilla. I used a one shot potion and had both monsters beat. They then wandered in a level 8 Face Sucker and made it +10 Humongous. I then used the Polymorph Potion for the level 18 Face Sucker and then their last ditch effort was another wandered in monster (can't remember which) but it was around a level 10, then they dumped some +potions on it making it more than my +22 fighting power...until I pulled out the DOPPLEGANGER from my hand and I was convinced I had won. At this point we realized we got a little sloppy with cards everywhere, but I clearly was winning. Below are the "heated" questions/"what ifs" that followed:

1.Did I win, or did we resolve this massive combat incorrectly, my understanding is its a "back and forth" process of increasing and decreasing fighting power until someone wins or losses and resolving combats in order?
2.My friend realized he had a Mate card after I played my DOPPLEGANGER and wanted to "mate" the Level 8 Face Sucker (with the +10 Humongous) potentially making it a +36? Would that have been possible had he initially thought of that? Is it possible to go back after that monster had already been "resolved" and mate it after I played my DOPPLEGANGER?

I love this game and just need more help understanding when things get convoluted.

Last clarification. In the general FAQ's it covers "exactly what is a weapon" and explains to use common sense with items like the Singing and Dancing Sword. My understanding from the FAQ is that it counts as a weapon slot, but Erik Zane states in a separate thread it doesn't count as an item slot? Which one is it?

Thanks!