Yes.
The former. The same Rallies can't stack though (using 2 units with Rally Strength won't give you +8 Strength).
Just the main unit.
Just the unit.
Defender is meh. Deliverer is very useful.
No.
Yes.
Yes, apparently.
You should.
You can forge and rename Shout Out weapons. Legendary Weapons cannot be forged.
Everything the team has in its inventory (not the convoy though), including forged weapons. The exceptions are Legendary Weapons.
+10 Damage.
Not sure.
One for every time you beat the DLC, so as many as you want.
Yes.
Lethality/Astra with Rightful King.
Because Sumia's version of a certain scene appears in the opening. She has a CGI with Chrom entitled "Lovebirds". And if Chrom has equal support points with all his suitors, Sumia gets the highest priority.
Granted, this doesn't 100% confirm canon, and nothing will unless IntSys outright says what's canon somehow.