1/2 track mono and 1/4 track stereo are different. Might play the tape with less s/n ratio might crosstalk.Might play one side, might not play the other side.
Sorry to contradict you, but no. On a 1/4 track head, the left channel is channel 1 which is located on the upper edge of the tape. Unless the deck has a very bad problem there is no chance of audible crosstalk with channel 3 which is the centre lower part. The track layouts for 1/2 and 1/4 tracks, and their variants are reproduced
here, with the widths. Do the math and you will easily see that the left channel of a 1/2 track tape covers 0% of the right channel of the 1/4 track head, with a substantial safety margin. The symmetry of the channel layout also guarantees that it will be the same when you flip the tape. If not, the deck is out of alignment and you need to calibrate.
And on a NAB machine would require some degree of treble boosting [...] Then you'd just have the playback curve difference to deal with.
Again, no. Not to compensate for the EQ curve anyway. There is a difference between NAB and CCIR at 7.5 and up, but 3 3/4 ips is NAB, period. Even on CCIR decks. Check the manual of your Otari if you don't believe me. That's basic knowledge of analogue recording, and the standards are widely available to check what I say.
In case you still can't believe it,
here is a comprehensive list of all the EQ curves used in magnetic recording over the ages. The conclusion: no problem with NAB unless those tapes were a unique case of using the short-lived, long-obsolete IEC or DIN curve defined for 3 3/4 ips, but then (a) it is not at all what has been said of these tapes, here and on other forums, (b) how would they know unless it's written on the box, and (c) good luck on finding a deck using that curve. Should the case occur a parametric equalizer, not a mere treble control knob, would be the only way to properly compensate for the difference.
Open reel does not have the wide compatibility between formats which cassettes do.
Apart from the noise issue (about a 2dB loss in signal/noise ratio), there is no incompatibility issue in this case, not with what has been said on the multiple threads on this topic anyway. To the risk of sounding like a broken record, there is no point whatsoever in using a specific deck for a given tape. Even if the tape was nonstandard, you coulnd't know which deck matches it - most probably none!