If ARC gives you any issues (and it likely will, as it always has for me), you can use the optical connection. Anything coming out of the TV is going to be a lossy codec anyway (Dolby Digital, Dolby Digital+, DTS).
I'm not sure I understand why you are concerned about the TV's ARC or optical audio out to downconvert to 2.0 audio if you are connecteing a 2.1 or 3.1 soundbar. Most soundbars come with wireless subwoofers. As long as the sub is nearby and not in the opposite end of the room, you should be OK...