I will try to answer your questions. I have only one precautionary engine shut down in the DC-9 that is several years old, and never had to cycle into reverse onto the landing roll since we were light.
I didn't say you would use the deployed reverser to offset "yaw", you would apply opposite rudder for that and nosewheel steering when it became available. But the bucket, since it "theoretically" will deploy, would have a drogue chute effect, if you will, to offset the assymetrical thrust condition. In other words, that may help the rudder deflection, all that much more, in keeping the aircraft close to the centerline. Will it be appreciable? I haven't been in a situation to make that call--and hopefully won't be soon.
But, this discussion comes up due to an incident we had in Denver (Stapleton) in 1994 before I was hired. A fully loaded -9 lost an engine at V-1. Crew and pax just heard a load bang. The force, they later found out, blew the reverser bucket off. The crew declared and emergency and landed three minutes later. The conditions were marginal and the runway was icy. Unfortunately, the plane came in a litle high and hot, and they had to go into reverse. The cycle into reverse caused the plane to swerve off the runway--just missing a drainage ditch. The crew recovered and stopped the aircraft at the very end of the runway. The crew was unaware of the full damage extent and did everything right, but did not know the reverser had blown off. The after-action on this incident stated that the offset drag, had the reverser been there, may have made the swerve less drastic. So, appreciable may be a bad term, but then again, in an emergency, I'll take every card I can play.
As for the 747s, the RRs deploy the reverse in a sleeve, not a bucket. Therefore you don't have the drogue chute effect.
I would be interested in hearing about the BAC-111 (Tays???) and its accumulator. It would seem that the ability to deploy the buckets was good engineering on the part of Pratt and McDonnell. You asked for my input, I would be interested on your thoughts. Thanks.