I need to do some subfloor repairs in our upstairs bath, the plan is to ultimatly lay tile down. Few quick questions:

1) The framing is 2x8's on almost 16" centers (they're all sistered and range from 16" to 20", it's hard to explain why other than "it's a split level")... I need to drop in some blocks along the wall to support the edges of the subfloor patches... do they need to be 2x8's as well, or can I get by with something smaller since they aren't structural just supportive?

2) What's the best way to support the blocking? Should I use big old joist hangers, or is there some magic framing component I missed last time I was at the store?

3) Assuming I have 1/2" (maybe 15/32"?) plywood as the subfloor... can I / what do I need to do to beef it up to support tile? I know backer board does provide any rigidity, so I'm guessing I need another layer of plywood... but what thickness?

Thanks!