IMO she spends only 10 minutes there because it's too cold. Plus, it could possibly be too early for her digging response.
As Susan says, use an under-tank pad. Dig down and check it every other day to make sure it's not too hot.
I would not add much extra water unless the mix gets too dry to clump - that is, it needs to hold a tunnel without spilling apart, yet not be sloppy-wet.
The calcium in calcium carbonate is utilized by iguanas, but it's downside is that it neutralizes their stomach acid and probably has other effects in the digestive tract. It important to not tip an ig's digestion process. I would get calcium glubionate. Some cheap calcium supplements are of calcium phosphate, like egg shells or ground oyster shells. I don't think this can be used at all by iguanas or humans.
Calcium glubionate is the usual solution to low serum calcium in humans and iguanas. It's cheap, used to be OTC, but now needs a prescription. I'm sure your vet will be willing to prescribe it without a visit, since her calcium has already been measured by her. You would only have to agree to take her in two or three weeks from now for another calcium check. Too much calcium causes bad things, including huge bladder stones.
Look up iguana bladder stones on Google :-(
I think everything is going great for your iguana!
Her low appetite is normal, even though it seems way too long to us. The diameter of the base of her tail will shrink due to the fasting, and is a normal outcome of gravidity. They recover nicely from normal gravidity.
If feeding her the slurry freaks her out (I don't know, she may be OK with it, and thus be okay to keep doing it), it's unnessesary(sp)stress and you may go ahead and stop doing it, with no fear of causing a bad outcome. The real need to force feed may come later if something goes wrong or she never starts to eat after this season she's in is over.