I know this is a fundamental Magic question that doesn't necessarily pertain to Arena, but I encountered a situation today in Magic Arena that I wouldn't ever have in Paper, so I just wanted to make sure everything was working correctly and if those questions can be answered.

To set it up, I have made a recreation of the events in MS Paint that you can view here: https://imgur.com/QDOgCtP (Replace "Severed Strands" with "Vicious Offering", my mistake)

I will go over EVERYTHING that happend that turn, in order as well, for as much clarity as possible.

The Board State: My opponent controls 3 creatures and has four cards in their hand. He has 12 land on his side of the board, four of it tapped, including an untapped & flipped Arguel's Blood Fast (Temple of Alcazotz). I control 12 land, 1 creature (Carnage Tyrant), and have Island, Forest, and Entrancing Melody in my hand. Neither of us know the contents of the other player's hand.

Upkeep: I untap my lands (3 forest, 1 island).

Draw step: I draw Departed Deckhand

Main step 1: I tap an island and forest to cast Departed Deckhand. Once successfully cast, I activate his ability (Paying UGGG) to give Carnage Tyrant "Can't be blocked this turn except by Spirits". In response, opponent casts edit Vicious Offering on Deckhand, which triggers Deckhand's self-sacrifice ability. I play no other cards and head directly into combat.

Combat step: (There is no pause, I'm immediately launched into declare attackers). I declare an attack with Carnage Tyrant and turn him sideways. Again, with no pause, Carnage Tyrant lunges forward and damage step just flies by, setting his life to -1. The game ends.

My questions:

At what point could my opponent have chosen to use the ability of Temple of Alcazotz? During my Draw? When I cast Deckhand? When I used Deckhand's ability? Its clear he didn't want to use the ability until he had to, but it seems he never had the chance to use it. Was the Deckhand Stack his last opportunity to use the ability? I want to know every exact opportunity my opponent could have used it.

Is the declare blockers step skipped if my only attacking creatures are unblock-able?

Would my opponent had more control if he had enabled full control? Or was priority simply never in his hands after the Deckhand stack ended?

XXX (EDIT: The card was Viscous offering and not Strands. My fault.) I'm fairly sure I'm not mistaken, but my opponent didn't sacrifice a creature for strands? I thought he had to do that as he cast the card. Did targeting Deckhand cut in and make it fizzle somehow?

I hope this was clear and that I posted this in the right space, I would love to understand exactly what went on, I didn't have time to take a screen shot because I was on the victory screen so quickly, so I hope my MS paint version is adequate. In paper magic I have never encountered a situation like this, so I wanted to be sure there were no mistakes in how the system handled these situations.