Hmm… While a reasonably full understanding of quaternions might be useful, I’m not sure that it’s important to your immediate problem, so don’t let the maths scare you off just yet.
Regarding HPR versus quaternions, if you take the approach that I outlined I think that it would likely be better to use HPR only for the initial step of setting the relative rotation for the sub-node; the lerp in particular may be better handled use quaternions alone, and the final assignment can easily enough be handled via the “setQuat” method (if I recall correctly), obviating the conversion to HPR.
(As to lerping between quaternions, take a look at this thread–it’s actually fairly straightforward. (But note that you should probably place the multiplication by “t” after the “(B-A)”.))