You may find the answers to some of your questions at the French website for the inland waterways at: Voies navigables de France
Also the UK Cruising Association website
had lots of information on the cruising the canals back in 2003 when I was planning such a trip. It appears now you have to join the CA to access that information (not a bad idea). Also they have a cruising guide for sale
on the website.
I abandoned my plans for the trip in 2005 when the control depth
of the canals got too close to the draft of my boat, 1.6m (5.25'). You need up-to-date info. The depth
in the spring is generally greater than that in the summer months.