"One of my questions is about possible damage to a guitar by stringing it with mediums and changing tunings frequently"

well, it never seemes to bother Don Ross, James Keelaghan, Stephen Fearing, or any oghte other folks who I see tuning and detuning all the time... Stephen swears by medium guage strings on his Manzer...

If you're being advised to notuse anything heavier than light guage, that'd lead me to suspect that the bridge is lifting or something... which shouldn't be an impossible task for any even half way decent guitar tec. to repair...

If it is unreparable, and yer looking for good affordable guitars, log on to Lasido.com

That's my advice for guitars... but I'm a little biased maybe...

Good luck eh!