The interaction between Aspasia and Metatron was SUPER well written. I tend to have someone else when I run across Aspasia but I had Metatron this time around, and I was half expecting "FIGHT!!" given how Aspasia and Metatron were responding to each other - more so after Metatron put herself between me and Aspasia, like "you want to fight him, you need to go through me first". Ah the perils of an archangel that hasn't fully awoken all her emotions yet and a predator slime that doesn't totally understand humans!
Trying to think of a possible Megatron cameo and the only thing that comes to mind is Megatron berating Starscream ALL THE TIME. Don't think the MC would berate any of the core group that way, expect MAYBE Sam. Also, I think you are giving Sam way too much credit on how much tact she has. Yes, I like Sam, a lot, but I do think a gorilla has more tact than Sam! I think Sam has about as much tact as a bull in a China shop, myself...