[kb2vxa]

"As suspected, terminating the unused input port is very important."
Yes it is, terminating unused lines/ports brings the SWR back into acceptable values which reduces or eliminates signal leakage via common mode currents.

"So is not chaining a bunch of barrel adapters."
I sure wish we had a Snopes for this sort of urban legend, nobody listens when this fallacy is explained.

"F-F male is just as bad as any slide-on connector when it comes to leaking 800mhz especially."
I talk to the wind.

"So essentially I'll use this when needed as either a high-pass or low-pass filter hinging somewhere around 300mhz center with high-quality direct connections and the unused tuned port terminated."
Sounds like a plan.
Now if you're resourceful you can make your own terminations out of F connectors, 1/8W metal film resistors and pot them with epoxy making neat, waterproof, and professional looking little units.