Is the car's stalling after the Istikhara prayer an indication that there is no good in the matter, or is it merely an ordinary incident unrelated to the Istikhara, the outcome of which may be a definitive rejection?

What you mentioned is not an evidence for the matter not being facilitated, so do not pay attention to it or consider it an ill omen. The scholarly disagreement is about what the person seeking guidance (istikhara) should rely on: is it the comfort of the heart and the facilitation of the matter, or proceeding with it unless Allah diverts him from it? The preponderant opinion is to proceed with the matter unless Allah diverts him from it. So, go ahead with your intention to marry this girl; if it is good, it will be facilitated, and if not, Allah will divert you from it.