No fuel in tanks except "unusable" equals exhaustion
Not necessarily. Depends on the aircraft configuration. The idea of the unusable tests is to cover modest turbulence. Often, you might have a bit more in reasonably balanced, smooth flight. You can't plan on it but it might just help in suitable aircraft if the aim is to get that extra mile or two out of it.
Could a trainer get into a tailslide even with a full power stall? Without a sharp pull-up from speed?
Why would you be doing that sort of thing ? Possibly be useful to have a read of certification stall procedures for run of the mill aircraft ....