So I just built a summoner at level 15 with an edolon that is a hoverbike with the trapped soul of Squee goblin nabob the unkillable goblin from old school magic the gathering. Stats are:
ME with magic items : Str:8 Dex:27 Con:18 Int:12 Wis:8 Cha:20
Edolon with ioun stone : Str:18 Dex:28 Con:14 Int:7 Wis:10 Cha:11.
So i bought 3 heavy crossbows for us to weild. two in his 4 arms with multiweapon attack, and one mounted on his head (basically carried by me). We both have the crossbow mastery line. So I think that I get ridiculous amounts of shots. When i move less than his 60 fly speed I get 4 with my +2 holy at 19/19/14/9 then the eidolon gets 1 with each at 20 for +1 seeking shocking and 21 for +2 flaming. Assuming we don't move he can rapid shot taking -2 and getting 18/19 plus after this he gets 3 shots at 0/-5/-10 using any combination of the two crossbows. Thats 9 shots per round. Is that too much??? I hope my DM doesn't think so.
Truth by told, I'm not sure i'm doing the math right so let me know what you guys think.
Without mounted skirmisher, you get 1 attack whenever your mount moves more than 5'. Your mount would also only get 1 attack.
Actually, you can full attack with a full weapon while on a moving mount. The same doesn't apply to a mount, though - he still has to take a move action and make a single attack with a standard.
Multiattack doesn't apply to weapons. I don't know if there is an official ruling for a creature with 4 arms using 2 2H weapons. My ruling would be to take a -6 penalty per attack (as per 2 weapon fighting) and the second crossbow would only get 1 shot per round in a full attack action.
It has multiweapon fighting, which is, as you can guess from the name, for weapons. So that part works, at least.