I do not wear nail polish so can i apply a coat on top of the previous coat everyday for a week then remove and start over?

Yes that's how I use this product

Can i use this with opi nail envy

I do not know what opi nail envy is, but you can use with anyw nail polish, I use it with Essie polish . This product is great, my nails are stronger than ever, I have the best nails I’ve had in years. They are so much stronger than ever.

I use gel polish. Could i use this under my base coat. A few of my nails are not recovery well from a long illness. Is this a good option?

I put two to three coats of this and then two coats of polish.. I’m not sure about gel polish. I know that this has helped my nails grow and be srtonger.

Can I use this before applying gel nail polish on top?

I would recommend against buying this product. After several months of glue on's, I wanted to heal my nails. Sapphire peels off within a day or so. I would assume the same may happen if you apply polish on top.