No, this comes in the length listed.
Nominal wall thickness should be around .065" per NEC
Yes - if you remove the zinc coating. If you weld the zinc bad things happen
Yes. I would be sure to use watertight connectors to prevent water from entering the conduit system.
No, this is not the product for use as your have described.
Yes but it will eventually rust.
Not unless specifically listed for the use. A rigid conduit has a larger outside circumference and would not make a watertight seal unless specifically listed and labeled for the combination use.
I used gas pipe (black threaded pipe).
Typically, openings for conduits of this size have to 'knocked out' or drilled for connection to an enclosure. You can measure for the outside diameter for a connector and drill the appropriately sized hole for connection. With it being 1 1/4", I would drill a 1 5/8" hole for a good fit. To be more precise, you'd have to get an electrical hole saw kit.