I'm obviously not going to be able to get updates going forward until I use the new installers.
?? Why you said that ? You can still continue to get updates, the only exception is KORD V2, which requires a new installer to continue to get updates for it.
My questions are.... What is the correct installation procedures? Do I run both the GSX and GSXL2 installers? .....
There's no such thing as GSX and GSX L2 installers, as if they were two different things. Since GSX L2 has been released, there always was a single GSX installer that contained both, nothing has changes since GSX L2 came out.
Well yes, there are two installers, one for 32 bit simulators ( FSX and P3D 1, 2 and 3 ) and another one for 64 simulators ( P3D 4 and 5 ), but GSX and GSX L2 has always been in the same installer.
Do I have to uninstall my version or can I run the installation on top of what I currently have? .....
As long you continue to use only P3D 4.5, there's just nothing you have to do, except updating KORD V2. AFTER you'll install P3D V5, you'll have to download and install the new GSX installer, which does what the updater doesn't, which is enabling GSX into the new sim.
Has the DX12 code been added to allow signage objects to work?
Not yet, we'll start working at it after the next major update to GSX, which will feature jetways support for AI. The DX12 code is very complex and potentially dangerous ( it's the only thing in GSX that *CAN* cause crash to the sim ), so we need extra time to do it right.