CMOS Checksum is often due to the CMOS battery dying or being removed. They typically last ~7 years or more so i'd doubt this to be the case. Sometimes they are wrapped in heat shrinkwrap with 2 contacts going from + - on the battery to a header on the board, possibly got bumped or unplugged? Also suppose it could have been dead/disconnected the whole time and removing the laptop battery cut the power.
If you're really at wits end, check for a teardown vid on youtube, Often times when I get stuck I'll look for a manual on their site (DELL often has, ironically, excellent "how to replace this" manuals) listed right on the support page, often under "Manuals"
That said, I'd expect to find the little tabs/screws labelled on higher end laptops by a tiny pic of a keyboard, and on any compartment cover that houses the aforementioned there should be the same symbol. If you don't see those, it's a crapshoot.
In your particular case, however, it can be tricky to diagnose. If you're getting weird issues with your KB, Touchpad and random CMOS checksum errors, you may have an issue with your board itself. Laptops are such a one off item, if the board goes, it's rarely worth replacing. If this unit is still under warranty, I'd consider that before you void it further. If all you've done is attempt to take off the keyboard you should be fine.
EDIT: hooray for reading, the two places you've listed as not having teardown instructions is where i've said to go. Let me give it a go.
