1. I found out when the tools did nothing. I got very close until my controller were hitting the block and there was a reaction
2. It could be complicated I guess. You'll have to check both whether there is a block in front of your head and whether there is a block above that block. At this moment, "autojump" feature in this situation becomes "autojump" and "crouch" but not easy to visualize "crouch" in VR. There would be less issues if the avatar was one block tall but that creates more issues with other game mechanics
3. The only thing I have to add is that people barely move above their starting height playing VR in the vertical movement. I bet even if you get the detection down by half an inch, it would be fine.
4. Not really original idea on my part. Games like "medieval engineers" on PC did it before. If you want to be simple, make them a bit transparent? or just put in a block frame. It get the impression you want to be less arcadey than base Minecraft. "medieval engineers" was bit bit more like that. Slightly more realistic build simulator games.
Will check the discord ID. I'm a novice with that service at the moment.