so hard to tell. there are a lot of types of mosquitos. some of those alpine/snow kind can be horrid, but they also die quickly. last weekend of July in 2009 was barely tolerable, well, it wasn't actually--cept for on the top of eagle cap/when there was a breeze. it was just a 2-3 day trip with my father and fiance so we left a bit early the one day. mosquitos were actually a lot better lower along the lostine river than they were up in the lakes basin, for that one.
Mid-August 2008 was pretty bad on day one, but it got progressively better, to the point that 4 nights later we were barely seeing any in the same area.
that said, keep your eye on any TR's at nwhikers or pdx hikers. the wallowa reports start to come in in waves around this time..
as for horses, yes there can be a lot of them, in the sense that i rarely, if ever, see them anywhere else in oregon, so seeing 5-10 there in 5 days is a lot, but by and large any horse-camps tend to be in the valleys and big and away from where other people camp. I've also seen some folks with pack lamas there, a couple, they had about 6 llamas. i wouldn't let it deter you one iota.
the distances (two pan to minam lake) (minam to mirror) (mirror to glacier) (glacier to 6 mile meadows) (6 mile meadows to wallowa TH) are all quite doable. minam to mirror and mirror/moccasin to glacier over glacier pass are both relatively SHORT days, with a serious climb being the brunt of it. They will be nice days to relax. if it is hot out jump in the lakes--swimming is great, just be prepared to not catch your breath once you go into glacier lake. moccasin has some nice jump-in spots.
in conclusion, you should go there and enjoy the scenery, see something new, as beautiful as the north cascades are.