When I pronounce "um", the tongue touches the end of the palate (or it's the beginning?), almost touching the base of the center incisives teeth.
When I pronounce "un", even if the tongue stays almost in the same position, it never touches the palate.
No, it's the other way round (/n/ vs /m/)
"Regressive assimilation" phonetically/phonologically means that a feature of a following sound affects the preceding sound. The feature here is the "bilabial" quality of the sounds /p/ and /b/ which results in /umplambe/. Your enunciation thus gets more speed. If you prounounce /n/ and /p/, the enunciation takes longer.
An example of machine-gun Spanish which gives me a head-ache:
María Méndez: '101 preguntas para ser profe de ELE'
https://www.rtve.es/play/audios/un-idio ... 22/6463789