Can only help you with some, but here you go:
1) I got a set of snaps from Rinker -- cost about 5 bucks and 5 for S & H. Just call Rinker or email customer service. Conversely, you can have your canvas guy use standard snaps when he sews the straps on, etc.
2) There's a generic manual on the Rinker site (
http://www.rinkerboats.com) that you can download. As far as I know Rinker doesn't provide a specific manual for each model -- one of the adventures of owning a Rinker!
3) You can check West Marine, Defender, or Boatfix for other table tops and mount them to your pedestal. I'm taking measurements of mine so I can do something in wood to make a custom tabletop. The only two tabletops I've seen from Rinker are circular and oblong, and they're not that big.
4) If you've "really really" pumped out the head (I had a layer that wouldn't come out, which raised a stench this spring), then Babyboomer has a charcoal filter design that you can make -- search the site for "filter" and it'll come up. I had found that my problem was that the vent was slightly plugged (which finally opened up when I did the "really really hard" pump of the head), and it's fine now. Two other things to check -- make sure your waste vent doesn't have a kink or crack in it and check the waste line from the head to the tank -- I've heard of people taking a damp rag, run it along the line, then smell -- if it's funky, replace the hose.
5) Need to know more -- do you have an electric head? Vacuum head? Is the leak coming from the flush valve or elsewhere?
6) Try cleaning the shower sump -- there should be a box with a bilge pump in it down around your keel area, but I'm not sure of the location in a 342. After you clean it, I"ve been putting a little white vinegar down the drain every other month -- haven't had a problem with the sump yet.
7) Most likely eBay or overstock.com. If you want marine quality, try Marine Electronics Unlimited, Defender, or Boatfix.

You can likely get skins from Rinker, but it's sure to be more cost-effective to use that same canvas guy in (1) above to get higher-quality skins made to order.
Good luck -- sounds like you have some busy weekends ahead!