i don't need SLERP for my project, so it isn't a big priority. however, i would be interested in implementing it later.

some of those problems sound like problems i've had when multiplying quaternions the wrong way:
to rotate quaternion2 by quaternion1, use "quat_multiply(quaternion2,quaternion1,quaternion2);"
it might actually be worth writing a function "quat_rotate(q1,q2)" which basically uses quat_multiply to rotate q1 by q2, but without any possibility of logic errors on the programmer's behalf. the quat_multiply system isn't quite intuitive when doing rotations, but it IS how a multiplication actually works.

after that changing the multiplications, try again (though you appear to mention trying this change...)

i'm not an expert so i don't really see how gimbal lock would be possible with quaternions.

i've seen some slight jittering or inaccuracies with certain angles, which i've mostly been able to associate with my custom atan2 -- jcl said a more accurate built-in atan2 will be implemented in a future update.

i'm glad you're giving this a go i'm mostly using quaternions for custom physics (the need for angle-axis rotation instead of pan/tilt/roll).

let me know if there are any more problems or they still persist -- i've been doing a lot of stuff with them and havevn't had any problems more than slight inaccuracies (quaternions remain accurate, just the euler representation that's off) and small jitters

julz