1. Depending on what gearing you want you may not have the straightest chainline but as long as you aren't running your chain from your big chainring to your biggest cog the only problem will be noise. (this method requires you to leave the whole cassette on the bike (the gears you aren't using are your spaces)

2. QR is fine with singlespeed. Possibly fine with fixed (i've seen it done but not by me so YMMV). Your only problem with QR will be chain tension but it's not impossible to get good tension with a QR