you can't pick out certain quotes to reply to when they're in a quote nest
Not sure if this is different on mobiles but on my screen in the upper right-hand corner of the Reply composition window there are four icons:
Undo, Redo, Toggle BBCode, and Drafts.
If you slap the 'Toggle BBCode' icon it shows the raw Reply codes with the nested {QUOTE}{/QUOTE} etc.
Then it's a simple matter of trimming out whatever isn't wanted/needed.