Only PCB mounted with that plate. This PCB is very sturdy and has no flex at all, for me is perfect without plate. The poker PCB has more flex and the plate is welcome.
A word of warning, the holes in the PCB for the switch plastic pins are tight, this is good, the switched sit very well, but it takes some effort to put the switches in. So a plate will make things a little complicated but not impossible.
The Cherry stabs will have to be placed before the plate and before soldering the switches.
In general stabs are required for Space Bar, LSHIFT (not for ISO short LSHIFT), Back Space ( if it's on the number row and is 2u, standard one), Return/Enter, RSHIFT( not for the 1.75u HHKB style RSHIFT).
For the flashing I will test tomorrow, it's a standard procedure for a programable keyboard, the risk of bricking is small.
For us is a little harder because we don't have a procedure but we will see how it goes
However the board has a default layout, so you can play with the board without flashing it, and is a nice layout. If you look at the links in the OP you can see the default ones.