I have an ELITE throttle quadrant ( TQ ) type MEL *snip* It does not work with FSUIPC.
The fact it does not work with FSUIPC leaves me suspicious about the throttle itself. FSUIPC reads any axis available on the computer, and you can utilize the axis in multiple ways. If FSUIPC does not read it, it is certainly possible Windows itself doesn't read it unless it is using the ELITE software itself to control the axis.
I had installed another version of the P-51 ( not by A2A ) and it works with that, too.
Accu-Sim is not using a standard throttle system, so unless it is mapping directly to the standard FSX throttle through it's software, I would imagine there could be possible conflicts. Most other addons do a direct mapping, however the throttle in the P-51 is nothing more than a manifold pressure regulator handle, meaning that it does not directly control the engine but rather tells the engine what you, the pilot, want.
So, there must be something that is incompatible between the ELITE system ( hw/sw ) and A2A's product.
Can someone from A2A offer any advice ( or any other person, of course )?
I'm not at my computer right now, but you might want to see if you can map it through FSX (which I am sure you have tried) or perhaps ask the ELITE support if there is anything they can help with. I'm not familiar enough with this, but have heard of this particular hardware/software combo giving people issues in this, and other simulators.
*snip* ....and neither the throttle nor the Prop and the Mixture levers in the aircraft are moving when I move the lever...
You could try the A2A configuration tool located in your fsx/a2a/p51 folder and see if you can get a response from that. If that doesn't work, you might want to Google for solutions, search this forum using the search feature and/or wait for someone who has experience via FSUIPC or this ELITE hardware/software.