RD hangers have changed very little over the years. Back out the “b” bolt and try again. As for chain retention, a clutched RD is the answer. They have far more spring tension than non clutched RD’s, in addition, the clutch tends to maintain chain tension and eliminate chain slap that we all know too well.
What range is the cassette? 36-11 for instance nets a below 1:1 ratio in the lowest end of range. Climbing is hardly impossible with the later wide range stuff. To the LBS and check their bins for used 10 speed stuff that can be had on the cheap! People think they have to buy into the latest to attain “Unicorn” status and discard the 10 speed stuff for 11 or 12 at an alarming rate!
Climbing with my one speed automatic works and usually a geared rider is what is being passed on the ascent. My one speed automatic has a ratio of 2.2:1 so, not extremely short gears but not excessively tall either. Since I spin a stupid high cadence on the open flats, I can easily taunt a roadie on a bike path! It ends to piss em off being passed by a singlespeed steel plusser!
Again, LBS’s takeoff bins are a treasure trove of 10 speed parts! At silly low prices, bonus!