Welcome to the Amel Family. I've recently joined myself and yes these boats are a steep learning curve. As they are designed and build by French people that have a strong opinion on what a boat should be, you can expect all kinds of unusual stuff. But, for me, most of it makes good sense. I'm sure you will have the same experience.
To your two concerns.
A rusty keel. Have it media blasted and re-coated. Anything else is just temporary.
Shut off valves are a pain in the $#%@. You never need them untill you do and then they don't work anymore.
The best but highly annoying solution is to exercise them every week. For you current problem you will need to remove all the diesel and renew the valve. But to a positive note that also gives you the chance to clean the tank. I'm sure it has collected some dirt by now.