If you want it to taste anything like the same, no. However, if you like Guiness more than you like wine, that may not be a bad thing.
Wine is generally slightly higher in alcohol content than beers or stouts, and that could impact how much of the other marinade ingredients get dissolved and how well they penetrate, but it's not a huge difference and I wouldn't expect that big an impact from it.
you can and cant
Yes and it will work well as both a pre-soak and a marinade.
sherry
Sometimes.
Marinade is savory for flavoring foods, and acidic for tenderizing.
Once marinade has had raw meat soaking in it, for health safety reasons it's crucial to bring the the marinade to boiling if you wish to use it as a sauce. You could pour it into a saucepan, bring to a boil, lower the heat and then add ingredients (such as a splash of wine or a little cream or butter) if desired. Simmer sauce to reduce it to the desired thickness.
It will change the taste.
No, because moscato is a sweet white wine, and marsala is a dry red wine.
You probably can, but keep in mind that port is much sweeter than red wine.
You can make stir fry with pretty much anything you want.
You can but it is stronger in flavour so don't use as much.
salad or marinade