One thing I can say for sure - do not use tapered threads just like Raven said, they will fail.
Using a fire extinguisher is a bit of a budget way to go and there are several degrees of dodgy-ness involved with connections, filling, diptubes, handles/taps. I use one myself and have never had any problems with it. Maybe I'm lucky but I feel I have enough knowledge and sense to know when to draw the line, some people don't. The problem with common sense is, its not always that common!
I've used mine with the dip tube in but will have it removed at the first refill.
I suggest you read and search on here a lot. The most important thing, IMO, is if you can't get an adaptor to make a good, safe fit between the reg and bottle then splash some cash and get a proper adaptor machined up.
L-bomb: I have no idea TBH but like you said, the relatively infrequent cycling is most likely not going to be an issue. The pressure test certification/stamping) would expose any weakness anyway I hope.