Answer to your first question: This is not possible. You can’t copy an existing preset into part of another preset.
Answer to your second question: I don’t use snapshots that often, but I have noticed the same behavior when copying blocks from other presets into different snapshots on a new preset. I was actually trying to consolidate multiple preset amp block settings into one preset using different snapshots. I do believe you can change the block parameters in the snapshots, but copying individual blocks from other presets kept overwriting the settings on the other snapshots that I had saved previously.
I tried this last night and actually raised this question on another thread today.