To answer your first question, I can only comment on Redgard, since I have not personally used Mer-Krete. Redgard is an excellent product but I cannot recommend using it only over existing cracks as new cracks can develop at any time. Redgard is designed to cover joints up to 1/8". It is not common to find cracks larger than this. If you want to save labor you could use a one step thinset/antifracture combination mortar such as Laticrete which also protects agains cracks up to 1/8". Here is that link.www.laticrete.com/portals/0/pdf/LA...
If you did need to cover a crack larger than 1/8" I would recommend using Crack Buster Pro crack prevention self-adhering underlayment which protects up to a 3/8" horizontal crack.
Most polymer-modified thinsets will work fine over Redgard, including Laticrete 253 Gold.
When tiling in any bathroom floor, always use a tile with a matte finish as opposed to high gloss. Even matte tiles can be slippery, so always use throw rugs near the shower or tub. Installation technique is the same.
There are many excellent grout sealers available. My favorite is TileLab (Custom Building Products) SurfaceGard penetrating sealer. Make sure you use a penetrating sealer and not just a finishing sealer which just sits on top of the stone.
Michael Prachniak - Expert Tile and Stone Consultant and Writer, Owner - Tile Productions
Check out FAF''s tile section!