Service Technician since late 1980s
That resistance from the center usually means a screw has been missed, double check that all of the screws have been removed. If you are working on a different model than the one pictured there could be additional screws hiding somewhere.
I don't have access to one of these right now, the one used for the guide was in for an upgrade, but looking at my photos it looks like there is a metal plate under the mainboard that holds the keyboard in place and it looks like it is held in place by melted plastic.
It that is the case, the official method would be a replacement assembly that includes the keyboard the retaining plate and the cover plate that the keyboard mounts through, moving all the electronic bits over.
The MacGyver method, (if you can get a replacement keyboard alone) would be to remove all the parts that block the keyboard retaining plate, break it loose to remove the dead keyboard and use glue to reattach the retaining plate.
If you have a heat gun (SMC rework station) you may be able to use it to soften the plastic retainers during removal and smash them back down later with a soldering iron but that basically never works.