A road mech swings at a different angle to an MTB mech thats why they foul on MTB cassettes in the bigger cogs, you can run an MTB short cage mech with 1×9, but thats loads more expense the long cage you have will be fine but run chain as short as possible.
To stop the jumping on the front its the MRP 1×9 device or a n-gear jump stop and a bashring.
If you ride offroad with nothing it will drop the chain, my record was 13 in one 1hour 20 min race, I bought a jump stop and bash the next day, they didnt make the MRP thing then.
I did exaclty the same as I can quite happily do my local loop SS or 1×9, but after a few years **** about being a bit niche, I have gone back to having the full 27 gears, its no more hassle, and I didnt find running less gears a great deal less expensive. (ok singlespeeding was a bit but still needed a chain and front ring every now and then)
poppa, I was using an unramped chainring when it dropped the chain 13 times in one short race, thats all rubbish as far as I can tell.