Is the ghusl valid if I doubted water reaching a part of the body after completing the ghusl, then had a minor ritual impurity (hadath asghar) before completing the washing of that part, and then washed it with the intention of removing both ritual impurities?

Successive washing in ghusl (definition) is a Sunnah (definition) and not obligatory according to the majority of scholars. Doubt after completing ghusl is not to be considered. As for one who is certain that he forgot a limb during his ghusl, he must wash that forgotten limb with a renewed intention, and if he had invalidated his ablution before remembering, he must perform ablution if he intends to pray.