1) No, the 990 can't do this one. The closest thing to that is the ability to reduce the subwoofer level automatically when in two-channel mode (2CH sub offset). For what it's worth, I have Studio60v2 speakers and an Outlaw LFM-1 sub, and I found that this arrangement (leaving mains small with the crossover at 60Hz and tossing in a 2 or 3 dB reduction in sub output for CD listening) works nicely for me - it might also work well with the Studio 20's.
2) Yes, the 990 does offer this.
3) Yes, the 990 can do this. Digital audio inputs can be assigned to any input, and you can assign one digital input to multiple audio and video inputs (I have optical1 assigned to both DVD and CD).
4) You'll need an additional amp for the surrounds and zone 2 speakers (presumably a Model 7125 since you can get a package price discount with the 990), so it will be a little bit of a case of apples vs oranges from the get-go. I haven't heard the 3805 to know how it compares sonically, but the 990 has really impressed me - I would tend to think that it would at least be worth trying out with Outlaw's 30-day return policy.