There are several different stains available all providing different finishes and protections. Transparent stains offer water protection but no U.V. protection due to having no color. Semi transparent stains offer both water protection and U.V. protection. These are oil based stains that soak into the wood coloring the wood but allowing the natural grain to still be visible. Semi solid stains are similar but with a little bit more color hiding more grain but not all of it. Solid stains are thick and opaque. They prove maximum protection from both sun and water but completely hide the wood grain. They also act more of a paint by sitting on top of the wood rather than soaking into the wood.

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the cons to solid stain is that it is prone to peeling when it fails and makes for a bigger job to refinish.

Stain is an oil based base that soaks into the wood. Sealer is more of a film that sits on top of the wood. Both provide water protection but sealer doesn't prove any U.V. protection. It also needs reapplied yearly vs stain which will build up over applications and last longer. typically 2 years.

No. You cannot apply a sealer on top of a stain. Both provide protection to the wood. Applying a sealer on top of the stain would be unnecessary and likely will peel off. You will need choose one or the other based on if you want color or to keep the wood looking bare.

This question is hard to answer without seeing the deck, but in almost all cases once you have a paint or solid stain on your deck you are stuck with it. In order to go to a semi transparent oil based stain all of the paint would need stripped and sanded off. This is an incredibly difficult thing to do and very time consuming and often not cost effective.