A good glass-mount 800 antenna might actually work better with your 996 than a NMO-mounted "pigtail", depending on the RF density at 700/800 in your area. Remember, scanners have needlessly-hot receivers without the necessary selectivity to make it effective, and the 996 is no exception. My personal experience has been that a low-profile 800 transit antenna works worlds better with a 996 than a wideband scanner antenna or 800 "pigtail." Of course, I'm also making the assumption that you only need the scanner for 800 trunking reception.
The term you are looking for in regards to poor glass-mount antenna performance is "capacitive coupling loss." Google it; I'm sure there are plenty of articles out there.
I'm curious as to what you need the antenna combiner for, since it looks like you're going to have a 1:1 antenna:radio ratio, except I don't see a UHF radio in your list. If it's to feed individual monoband V/U/8 antennas into your scanner, I would strongly recommend ditching that idea and going with a unity-gain multiband antenna since any resulting gain from a multi-antenna setup is going to kill the front end of your 996.
What kind of vehicle is all of this going into? I'm guessing a SUV since you didn't mention a trunk, in which case you could always use NMO L-bracket mounts off the hood. Be sure to use a good double-shielded cable like Times LMR195 or LMR200 if you go that route to help keep ignition noise out, and make sure the hood is grounded with braid to the body.
Good luck, and be sure to post pictures of the finished product.
Any material on the 'net or something I can use to to prove my point, somehow, and not just "cuz I know it's gooder."