Morgan Bateson. Get Kelsey Grammar back to voice the character. Lots of Boomer style jokes about how he is from another time period.
Potentially, yes you can, if you treat captain the position different from captain the rank. As can happen, at least by regulation, in the US Navy.
Ensign is unlikely, but as you say legal. IIRC, the minimum CO rank in the modern USN is Commander, but the USCG does have several small vessels (broadly analogous to the Raven-type down to a runabout) that have LT-JG or LT as CO, usually with a senior PO or Chief as 2-i-C.
Yes, I do find it extremely unlikely as well but still legal within the regulations, from my layperson's understanding.
I would imagine that it being legal has as much to do with manpower requirements of watchstanding (mostly led by ENS to LTs AFAIK) as anything else.
Thomas Halloway When Picard confronts the Nausicaans, Halloway gets Cali class instead of the flagship XD
I would too, though Starfleet has had a history of struggling with line officers vs. support officers. The US Navy and Coast Guard make more a of distinction beyond just rank.
Starfleet has that the same distinction, it's just that they aren't locked into one or the other as is mostly the case with naval personnel (The Army and Air Force don't segregate to quite the same extent and movement is possible at least in the same direction as Starfleet allows)
Your quote made me think of this for some reason: https://www.wearethemighty.com/migh...ut-star-trek-after-enlisting-in-the-military/
Given that Tilly isn't wearing red in the teaser but some-else who seems to be doing double-duty as Science Officer is, then it's almost certainly that Acting XO Tilly isn't going to be a thing. XO Saru might be a thing, but given that the only shots we see of the XO are an out-of-focus shot on the bridge and a shot of them and Burnham taking each other's hands, then IMO there's a non-zero chance that it's a new character that they don't want to spoil.
I think Saru is probably going to remain a Captain on duty with Starfleet Command (with maybe his own ship) who joins in to assist Burnham during the latest emergency. They went through all the convoluted trouble to make Tilly an XO, I don't think she's going to have that taken away.