We'll be in Rome June 19-23 and plan on getting the Roma Pass. I
believe that includes entry to the Colosseum/Forum/Palantine Hill. Do
we also need to reserve a timed-entry spot?
Yes, you do for the Colosseum, Your best source of information about the pass is the Roma Pass website:
https://www.romapass.it
Info about making a timed entry reservation for the Colosseum is on the FAQ page, #7.1: "For which museums it is necessary to make a reservation? How can I book?"
https://www.romapass.it/en/FAQ/
Also, just saw posts about the underground tour and how difficult it
is to get tickets. Is it worth it to try?
If you mean the Coopculture 24.00 € or 34.00 € tours which include the underground, they are extremely difficult to get during high season; usually selling out within minutes of becoming available. It's easier to try to book a tour with some of the trusted 3rd parties often mentioned on the RS forums, such as Walks of Italy, Roman Guy, etc. They are more expensive but some of them include escorted visits of the Palatine and Forum as well; complex sites which benefit from a knowledgeable guide.
Coopculture does not currently offer a night tour but some of the indie companies do, such as the two mentioned above. I've not taken one personally but it's another way to get into the underground if determined to do so.
