evening visit to Mont Saint Michel

The Abbey's web site indicates the hours in August are 9:00 am to 7:00 pm. Whether one can stay on after 7:00 it does not say. Since you are staying at a hotel on the island, see what the hotel can tell you.

You may be confusing the daytime access to the Abbey, which ends at 7:00 pm, with the Night Show, which runs from 7:30 to midnight. I don't know if anything is posted that directly relates to 2019 but here's what the Abbey posted for 2018 (which indicates that the same information applies to 2019 as well).
http://www.abbaye-mont-saint-michel.fr/en/News/NEW-ABBEY-OF-MONT-SAINT-MICHEL-NIGHT-SHOW-2018
"THE CHRONICLES OF THE MOUNT, the new Bruno Seillier’s creation, will take place over the next 3 years. This year, the first chronicle “AT THE BEGINNING” is performed from June 7th to September 1st between 7:30pm and midnight, every evening except on Sundays."
The evening experience is different from a daytime visit (whether self guided or with a tour guide). Can't say one's better than the other, they're just different.
The daytime visit will give you more information about the Abbey's history, construction, usage, etc.
The night show is exactly that, a show, with music and lights, some lasers as well. You see the show as you walk through the Abbey. They keep it fresh by re-designing it from time to time, as the above excerpt says. We've seen two Night Shows. One was exceptional, one was just okay. Obviously your reaction could differ.
I can't imagine that you'll regret it, whether night or day.

Bob is right. We visited in the evening, arriving at 5 and walked throughout the abbey for two hours. It definitely closed at 7. But those two hours were nearly empty. Perfect time to see it.