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 1x9, 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 1x9 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 1x9, 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.