I think having the case sealed is likely a good way to discourage idle tampering that could affect the warranty or FCC certification. Note I’m not suggesting what you’re up to as being idle tampering. Far from it, I’m looking forward to your poking around in there, but manufacturing a mass market device tends to require building things to address mass market problems.
I understand your frustrations over the RF shield. Cracked mine open with the Land Cruiser, but that darn shield managed to protect the PCB while only breaking loose at a few places. Removing it be desoldering the numerous legs that hold it to the PCB somewhat aggravated an intermittent contact issue in the area of the Bluetooth antenna’s emergence from underneath the shield.
It’s probably also good to point out that your pics, although connected in your text with a desire to swap batteries, actually show the opposite, antenna end of the goTenna. This pic shows the reverse side of the whole enchilada with the battery connections at the opposite end from the antenna.
The red and black battery leads are soldered to the PCB in the second pic. They are easily accessible on the other side of the PCB to desolder – once you have the case open.