so many questions...so little time
1. audio source to receiver, yes you would lose TV sound level function.
1a. yes, the receiver would need the feature "Dolby Volume", a sound leveling technology. It's is not available on all a/v receivers.
1b. if you feed audio to the TV and then feed it out the optical port, you would probably get a stereo downmix....and lose the DD5.1 capability. The DD5.1 decoder is part of the ATSC over-the-air tuner for broadcast TV capability, or the QAM tuner for cable TV....only this DD5.1 output is sent to the optical out connector on the TV.
2. For most entry level blu-ray players, YES, the receiver would have to handle audio over the HDMI cable and have the HD audio decoders built in. Some more expensive blu-ray players have built-in digital audio decoders and output to 6 or 8 separate analog pre-outs. Your receiver would have to have a provision for direct analog channel inputs to marry up with the blu-ray player.
3. no real comment...."usually" the TV upscalers are more than adequate for the purpose.
4. there is no way to feed 6 or 8 channel decoded analog to a TV, the TV only has a stereo pair of speakers, so what would it do with the signals ?
For a true home theater sound system, you need the video and audio "sources" feeding an audio/videl receiver. A single HDMI cable from the receiver to the TV feeds video to the TV.....the TV's internal audio is not used. The a/v receiver than decodes the digital audio sources and sends the amplified signals to a 5.1 or 7.1 speaker array