(Two bugs) Huge mushroom glitches

Simplifying this ticket to the most prominent of the multiple bugs posted here. It is much easier for us to deal with bug reports when they contain only one bug. Feel free to create new ticket(s) with additional bugs.

First part is a duplicate of MC-5474.

Currently mushrooms blocks are not supported, they are not available in creative and when placed do not preserve their damage value - feature request.