It is the solver itself that determines all these things (shape, SNR, HFR, etc)--at least the first half of the solver, which detects stars and then feeds the star detections to the plate solve code. All this is inside of the StellarSolver library. In any case, in my experience, even if we could do that, determining that the image has star trails etc wouldn't be that reliable and so occasionally waiting 10s for the solver to timeout is likely a better solution.
I just tried a real life polar alignment with the plate solving method using your suggested 'workaround', by setting an arcsecond-per-pixel value, instead of the arcminute width and the PA, and plate solving performed without any problem. I had "Refresh solver failed" errors before but not this time. Furthermore, the errors get updated correctly contrary to before where the errors would not always get updated and often not displayed.
William Optics GTF81 on Avalon M-Zero OBS
ZWO ASI1600MM Pro
ZWO ASI290MM (guide)
ODroid N2 running Ubuntu Mate 20.04 LTS
KStars 3.6.0 Beta
I noticed that there had been an update pushed out yesterday which included code changes as described above for the plate solving method. I've updated just now and will retry when the weather clears up.