Vic,

I'm no expert, but here are my experiences.  I've recently replaced a lot of the engine room hoses on Rascal due to rusted hose clamps, stiff hoses, and to simplify some non-Amel modifications.

1. I've been using various grades of Trident hose for fresh and sea water.  They are mostly transparent but reinforced with either fiber or a PVC helix.  It's helpful to be able to see whether liquid is inside, particularly when troubleshooting a bilge pump issue.
2. Doubling the hose clamps is dependent upon the length of the fitting that the hose slips over.  If the fitting is too short, adding a second hose clamp can do more harm than good.  If there's room, I've used two.  Be sure to look for good quality stainless steel hose clamps, preferably not the perforated type.
3. I don't know.  Rascal had a minimal number of valves in the water systems and I've kept it that way.  I added a lot of isolation valves when re-plumbing my house, and they weren't worth the trouble.  I replaced a couple leaking metallic valves on Rascal's water maker with PVC valves.  I'll be watching to see how well they hold up.  
4. A ratcheting pipe cutter or flexible tubing cutter will make your job much easier by giving fast, clean cuts:
https://www.homedepot.com/p/Husky-Flexible-Tube-Cutter-97642/304217583?modalType=drawer
https://www.homedepot.com/p/Husky-1-1-4-in-Ratcheting-PVC-Cutter-16PL0101-1/304217581?modalType=drawer
A heat gun is very helpful for both removal and installation of hoses.  I use a cordless one, which avoids combining AC voltages with work close to water:
https://www.homedepot.com/p/RYOBI-18-Volt-ONE-Lithium-Ion-Cordless-Heat-Gun-Tool-Only-P3150/306925921?modalType=drawer
5. You need to use hose that is reinforced so that it doesn't collapse under the vacuum of the bilge pumps.  It will be sold as bilge hose.
6. I ordered my materials online from Defender.  I wasn't able to use a local supplier, but I'm sure there are a few in San Diego.  The reason I've been using Trident hose is simply because that's what Defender sells.
7. I don't know.  I do frequent visual checks, and my family knows to report any water where there shouldn't be water, or any pumps running at unexpected times.
8. I believe there are some topics here on cleaning water heaters.  I haven't tried it.  I just replaced ours due to a slow leak in the tank.

-- 
Kevin and Elise Fox
SM404 Rascal
Charleston, SC