What is the ruling on the fiancé entering upon his fiancée at the engagement party that includes women (mahram and non-mahram), for the purpose of seating her, or putting the ring and jewelry on her, or accompanying her after the party ends, or taking souvenir photos with her during this time? And does the ruling differ if the attendees are only mahram?

If the girl is merely engaged: she is a stranger to her fiancé. Therefore, it is not permissible for him to touch her body or to dress her in anything. Escorting her to the hall or when leaving it is not free from Sharia (definition) prohibitions.

However, if the marriage contract (aqd) has been concluded: it is permissible for him to dress her as mentioned and to escort her into and out of the hall, provided that no forbidden act results from it.

It is absolutely impermissible to take commemorative photos with her. Photographic images are a matter of dispute [among scholars], and those who permitted them did not permit photographing women due to the potential for corruption therein.