Will a 2x with a clutch RD be just as effective as a 1x with a clutch and narrow-wide chainring on an mtb?
It will help 2x, though 1x is going to hold chain better. I think you'll still see an improvement with a 2x setup due to added chain tension. When I upgraded the drivetrain on my full suspension to use a 10spd Shimano Deore clutch derailleur it didn't drop chain nearly as much if ever. Make sure you have your chain shortened to the appropriate length as that's one of the biggest factors causing chain slap and jumps is having a chain that's too long. 

A 1x would likely work better if you switched to a narrow/wide chainring that prevents jumps though. I almost never drop a chain on the 1x setup with a race face narrow wide ring. The front chainring does a lot to stop the chain from slipping off due to the tooth profiles that really keep the chain engaged. I'm running a cheaper 10 speed setup on my hardtail with a sunrace 11-42 casette with a 10spd sram x-9 clutch derailleur and it works very well and was a cheap upgrade vs a whole new 11spd.