Blood Sun is irrelevant here - the message that you see in the client explains the exact problem. You're trying to assign 7 damage from Shatterskull Smashing, which is an amount of damage that Shatterskull Smashing can't deal. As soon as x is 6 or greater, Smashing's bonus rider kicks in, and it...
This is working correctly. Magus' ability to change the land types applies in layer 4, but Dress Down doesn't apply until layer 6. It's very unintuitive, but this is correct.
This is working as intended. The party mechanic requires one unique creature to fill each role in the party. It's well documented that playing a Changeling does not immediately give you a full party.
I have found a subsequent issue related to this that does materially affect gameplay.
When playing against a Tabernacle, I declined the first prompt to pay for my creature A. I then floated mana, intending to pay for my other creature B, but before I had declined the second prompt for creature...