Hi, I've stayed near Playa Del Coco the last 3 Augusts! I can answer your questions.
1) I've done Bats 6 times with Rocket Frog Divers, never cancelled, they go several times per week. This past year I was supposed to do a local dive for my last dive, the boat to Bats showed up and took me there instead for no extra charge. 5 out of 6 dives had bull sharks, one dive had 15 circling us for 40 mins
2) Local dives are generally 45-60 mins depending on the number of divers. Bats was less since it's deeper, around 35-45 mins per dive, also circling bull sharks will cause you to use a bit more air
3) Viz isn't as good as the Caribbean, but generally it's about 30-50 feet on average.
4) Local diving is pretty good too, lots of schooling fish, sting rays, eagle rays, white tips. Crazy amount of puffer fish too.
5) As stated earlier, no issues with viz. A couple years ago there was a red tide when I arrived and the viz was 3-5 ft. 2 days later it was back to normal.
6) Costa Rica has many distinct areas, having said that, I don't recall seeing any mosquitos in my 6 weeks there around the country.
7) I can't answer this.
8) about 4-1, I had 4 dives this past year where it was just me and the DM, they still went out for 1 diver so I don't think they cancel very often for not having enough divers. Typically, the bats boat had 6-8 divers, local 3-8.
I highly recommend Rocket Frog Divers, they have a shop in Flamingo Beach as well as Coco Beach. They will pick you up within reason as well. I've been to quite a few places diving, and this area is my favorite by far.
Here is a video I put together from Bats, Catalina and local sites from this summer:
https://youtu.be/F-oK0-Yw3iA