Is my wife going to kill me with this itinerary, Easter Sunday guidance, and other random questions

Overall this is good, but yes, your wife is going to kill you if you even think about doing the Kristinartindar hike over Easter timeframe That is one that typically is closed from spring til June as the thawing makes it unsafe, and of course winter is not the time to be doing this one either. The weather in Iceland is unlike anything else and the winds can and do blow cars off the road, so imagine being up on a spine of a mountain. I will leave it at that, as I think you get the idea that this hike would be insane;)

The good news is the rest looks mostly good!

Yes, Easter and that Monday many things will be closed. That is a very important holiday in Iceland. Nature does not close though, so just plan on being outdoors on Monday.

Your day in Reykjavik there is still enough to do. Walk to the top of Pufa, check out all the street art, Harpa inside and out, the Sun Voyager, the big church, go to a local pool (limited hours on Easter, but ones like Sundhollin are still open), do the food tour, do the Citywalk "free" walking tour (tip at the end), etc.

The Solheimajokull glacier hike is fantastic! You can meet them there at the Icelandic Mountain Guides location. Book ahead.

Katla Ice Cave is a great trip from Vik. We did this with Troll.is and had a great time! We had the cave to ourselves and the guides were encouraging us to take our time. You could also do the Crystal Blue ice cave with them, but that is from Jokulsarlon.

Again, the Mulagljufur Canyon is not a wise choice. The thing is, what we think of as spring is not spring in Iceland. It is still much more like winter. Spring sort of just skips Iceland, in a sense.

Selfoss makes the most sense for sleeping on day 7. Plus you are then close enough to do a few things before heading to KEF to fly home.

Note that you absolutely want to be at KEF at least 3 hours before your flight. Sometimes we get through fairly quick, but there have been times we were racing. It isn't security that has taken a lot of time, but rather passport control. So after security, there is an area where shops and restaurants are. You can get a bite to eat there, but don't hang out too long and just go to passport control when the line is minimal. Then go through there and onward to the D gates which is where all of the flights to the US leave from. Gates are not announced until about 45 minutes before, so then everyone makes a mad dash, and it becomes an absolute zoo. This will help you avoid that. Have the Iceland Air app on your phone as we almost always get an alert to the gate before it is announced on the boards in the airport.

There are so many amazing hot springs! The problem is always narrowing it down. We always try to fit in as many as we can on each trip. Hvammsvik is just north of Reykjavik and is on the way to/from Snaefellsness and so far it is hands down our favorite one in the country. In the Golden Circle, Fontana will be closed when you are there, but there is the Secret Lagoon which is nice, but a brand new one just recently opened, Laugaras. We are going there in a couple of weeks:) This is a higher end one than most. Then there is Sky Lagoon in Reykjavik and their 7 step ritual is heaven. The Blue Lagoon of course is great, and close to the airport so can be a good stop before heading home. As I mentioned earlier, the local pools are great too. Just check all of these out and see what looks best to you. We have done all of these numerous times, minus Laugaras which as I said we will be going to very soon.

Thank! I am wondering if staying closer to Jökulsárlón seems more prudent then as opposed to Svínafell since we most likely won't be able to do the big Kristinartindar hike.

Sure, that would work. Anywhere in the general vicinity is good.