Wondering if it's a good idea to get these tickets ahead of time. I usually like buying all my entry tickets in advance to save time, but with this system you have to select a time slot and since I am arriving the day I plan to visit the Duomo I don't want to be tied to a specific time. Are there usually long lines to get in to the Duomo and up to the terraces? If I did buy online, would they be strict if you show up outside of your scheduled time frame. Also, there are two ticket types that don't appear to be explained on the website: Pass Intero and Terrazze Intero. I know the second one is for the rooftop, but does that mean the second doesn't include entry into the church?
Yes, the Duomo pass allows you to enter the cathedral, the terraces and a museum, while three Terraces is only for that.
There can be long lines to enter, depending on the time out three day, so if you have the times reservation it could save you some time, although the security line cannot be skipped.
The ticket procedure is clearer if you look at these PDF links. The Fast Track ticket was until very recently an open ticket, valid to 12/31/2018.
Fast Track
https://www.duomomilano.it/document/3efaf704-e3c0-42de-9b70-c94df80d4b1f/
Hours and Admission
https://www.duomomilano.it/document/c1f3800a-0643-4e8d-a4b4-6362314e3a9d/