Is it permissible to intend both the obligatory (fard) and the sunnah in the three washes for ablution (wudu'), especially if there was a forgotten spot not reached by water in the first wash, and it was remedied in the subsequent washes, and does this situation necessitate redoing a wash with the intention of the obligatory?

If you intend to remove the ritual impurity at the beginning of ablution, that suffices you, and you are not required to renew the intention for each washing. If you leave a small part unwashed in the first washing, and it is washed in the second or third, if you do not intend it for merit, it suffices you.

Interlacing the fingers is obligatory according to the Malikis in ablution, and it is permissible in each of the three washings. If you interlace your fingers in the second or third washing without intending it for merit, that suffices you.