The best way to get here as there is not really any parking is to take the metro. The metro stations of Saint-Jacques and Mouton Duvernet are both very close to the location.
Irrelevant, underground, all of it. Also sells out constantly all year round so no real "best time".
No tripods, so bring a fast lens and I would recommend a lightweight camera. The skulls and area can get very dark. Flash is not allowed, but your phones led flashlight, if not overused can assist in lighting.
You can only visit if you have a reservation. Reservations can be found online and sell out fast. Typically they book only a week in advance so watch closely for when they become available to book a visit. The nice thing is that there is no guide and you can spend as long as you like. Hang back and shoot to your hearts content. Also, it should go without saying, but don't freaking touch anything.
This is a photographers bucket list location. Highly recommend. A highlight of our family trip to Paris! Get tickets here: https://www.catacombes.paris.fr/en
