Is it obligatory to interrupt one's ablution (wudu') and prayer and repeat them if intrusive thoughts and imaginations occur during them, or is that considered a whisper (waswasah) that should be ignored?

If you doubt the emission of something that invalidates ablution (like madhy, prostatic fluid) during or after wudu', then the ablution is valid, because the default state is the continuation of purity. Similarly, if you doubt its emission during prayer, the prayer is valid, and it is not permissible to interrupt it. However, if you are certain of the emission of madhy, then the ablution is invalid, and the prayer is invalid if it occurred during it. The best cure for obsessive whispers (waswas) is to ignore them.