It has proven and peer reviewed expert information. We are grateful that Melissa Kaplan chose to freely share this work with the general herp community. It explains how to make nest-boxes, and the facts about egging and lots more.
IMO Isa was within the bounds of normal behavior. I read about someone finding an overgrown well in the wilderness in Costa Rica, and found many iguanas hanging onto the vegetation down inside of the well, apparently to get a break from the heat. So with that and other information I came to believe that iguanas would use cool spaces if they had access to them.
Even so, when that behavior persists, the iguana needs a thorough physical and metabolic exam by a very good exotics vet. It looks like your vet didn't know the sex of Isa, so she didn't check for female problems. But retained eggs lead to a very prolonged illness, and the ig wouldn't eat anything at all for many months, which Isa doesn't seem to have had. You didn't mention if Isa got any antibiotics. They can cause temporary improvement, and when the prescription is used up, the ig goes back into the ill behavior (not eating, drinking, and no moving around).
After a couple rounds of antibiotics with no lasting improvement, the vet knows there is a deep issue, probably a deep infection of the ovaries, gut, organs, etc. Blood panels would show high white cell counts.
I can't even guess about the cause of Isa's fate. It's beyond figuring out. You meant well, and took him to a reptile vet. But the vet couldn't figure it out. You should stop blaming your vet, because that is bad for YOU. At the time, it was the best you could do. Stop beating yourself up. Many of us, including me, have lost iguanas to unknown causes, and have had to find true exotics vets even if they were 2 1/2 hours away.
If Spike's eggs are fresh and healthy, she can lay them. Sometimes igs just dump them wherever they are. Often they use nestboxes. Sometimes they need the oxytocin injections, and sometimes they need surgery.
Again, read this about egging and nest-boxes:
If her eggs have become hard and Spike is egg-bound, she can be cured of her retained eggs. She can have a hysterectomy. But it's delicate surgery because the vet can't leave even a tiny bit of the ovaries behind. The outside of the ovaries are connected to other organs and separating them needs very skilled scalpel work. Most hysterectomies are successful and the igs recover.
The retained eggs might already be grossly infected. That would show up in a blood panel. I don't know her white blood cell count and can only recommend that you find an exotics or a reptile vet who has done many iguana hysterectomies. No matter how far you have to drive.
Do read the website, because with knowledge comes power over your and Spike's life.
I also want to mention something about free-roaming iguana temperatures. They are down on the floor, or close to it if they don't have high shelves to roam on. If the room temp is 85F, at the floor there is air blowing under the doors and the temp there can be in the 60's, because of your cold climate. You do mention that they headed for the lamps (warm zone) when they were cold. When they stop doing that they are not thermo-regulating, and may be off of their food or water too. This condition requires a thorough vet exam to find out what is going wrong. It is possible that they only "looked" like they were in their warm zone, but even the warm zone was to breezy or cold to keep them healthy.
In the northern states, free-roamers are at a disadvantage and need very large solid-walled humidified topical terrariums to spend most of their time in.
So it's unavoidable: Spike needs to see an experienced exotics or reptile vet. This is something positive you can do.
I hope this answers your questions and relieves you of this stress you are in.