Wheel bearings and bushings

1991 Geo Metro, 3 cylinders, 51K original miles.

I replaced the bearings on the left rear tire. They failed twice. The first time, I installed the two piece set but noticed that there was no seal. Didn't think it needed one. However, they started scraping and then failed. The second time was just last week and I had a mechanic at Meineke replace the bearings. He said sometimes new bearings fail if not packed with grease blah blah blah. I am hearing the slight scraping sound again when I reach 50-60 MPH. So I replaced all four tires because they needed to be replaced. The guy at the tire place said the rim was slightly out of balance but he said not enough to hurt the bearings etc. He balanced the tire properly and put it on. He said, though, that my rear bushing was almost gone on that tire control arm.
I will need to get an alignment now that the tires are new. I got an estimate to replace both bushings. The mechanic said they have to replace the control arms.
To replace:
2 control arms- 185.14
Labor 1 hour-65.00
4 wheel alignment-59.95
total: $320.59

My questions to you are:
1. Is this a good estimate?
2. Why can't just the bushings be replaced?
3. Why do I need the whole arms?
4. Would this bushing problem cause the bearings to break?

ps. I have driven with the new tires and I do not think I here the scraping anymore. Is it possible that the old tire was inproperly balanced and that is what caused the tire to wear improperly and thus cause the bearings to break?

Thank you for your time and help.