Hello and welcome to JustAnswer!
Once the seat base was out you should have found to bolts that hold the bottom of the seat back to the floor, remove these two bolts.
There are more bolts at the top of the seat back which come in from the trunk side unfortunately, so you won't be able to completely remove the seat back without being in the trunk. You may be able to remove the lower bolts and raise the bottom of the seat enough to get underneath and to the trunk, at least enough to get to the keys.
If you're unable to get in through the back seat then you can have a key made by a locksmith or GM dealer. With proof of ownership either of these will be able to cut a key using the car's VIN and get you in. My dealer only charges $5 to cut a key in these situations.
Hello, unfortunately Cadillac had put in a liner isolating the trunk and the seating area. This liner is probably made of some poly material and is very hard to penetrate. The only direct access to the trunk area are two small openings on either side of the car and inaccessible to the trunk without having the ability to remove the back rest completely. So the only other option is to have a key cut with the information that you provided. But here is a question that I haven't been able to find an answer to. Is there a release within the cab of the car other than the trunk release button located in the glove box? Something that can be accessed with some ease?