You mentioned that some chords sound different when played on a 12-string guitar. On a 6str chord inversions are fairly direct, but on a 12str mixing in the extra octave pitches can confuse things somewhat.

Take the first position E7th for example: on 6str; E-B-D-G#'-B'-E" (E-B-D-G#'-D'-E") on 12str; E-E'-B-B'-D-D'-G#'-G#"-B'-B'-E"-E" (E-E'-B-B'-D-D'-G#'-G#"-D'-D'-E"-E")

The D is the 7th of the chord, and hearing the E'-D' 2nd combination buried in the middle instead of the widely spaced E-D-E" 7th or the D'-E" 2nd at the top is probably what sounds different. (Getting the intonation clean enough that all three D' pitches match can also be an issue.) Chords that include 9ths or 11ths can sound even more wonderful or weird. Try using inversions that put only the root, 3rd and 5th on the octaved base strings and keep the larger intervals on the treble strings.

With regard to dropping the whole tuning a step or two to save strain on the neck and bridge ... I've found that it really depends on the particular guitar.

DADGAD on a 12str is kinda fun as well, but keeping everything in good intonation can be a real bear.

~S~