I've had a few 12-strings over the years, but no "high-end" ones (think I'll go looking for an Appolonio if I get serious about this). Older ones did indeed suffer if tuned to concert pitch, but I want that Leadbelly BOOM instead of the Joni Mitchell twinkle (I almost wrote "tinkle" but decided it could be misconstrued), so even if/when I get a good 'un, I'll tune it down and use medium strings. Michael Cooney, for instance, not only tuned low and used heavier strings, seeking the Ledbetter sound, but has even used a clamp at the bridge to ensure good string-to-saddle contact when using a tailpiece-equipped guitar.
The old Stellas were made by the Oscar Schmidt Co. of autoharp fame, and were of better quality than the Stella by Harmony ones, which were basic music boxes for sure. That allowed, though, you can get good sounds out of even a Harmony Stella 12-stringer if you set it up right.
The poster who wondered about getting his guitar neck straightened should know that anything is possible, but sometimes it ain't worth the trouble. If it's an old high-grade guitar, or a collector's piece, like an Oscar Schmidt Stella, then, yes, I'd do it. Seek out a good repairman and be ready to spend $250. If it's a $250 guitar on the used market, I'd replace it. If it's a Framus, I'd set it down and back away slowly.
As for harmonicas, cross-harp is in the key of the 5th note of the harmonica's key (C harps play G blues, etc.), OR, from the song's perspective, the harp is in the key of the 4th note in the song (G blues requires C harps, etc.). If you forget which starting point you're using, harp or song, you'll get some really weird-scale blues.