What is the best solution to power up two 5292 buggy motors?
The Lipo, thermal protections kicks in when consuming about 1A, while the buggy motor will consume 1.3 A when loaded but not stalled.
So for two buggy motors, best would be 3 lipo's in parallel but perhaps you can get away with 2.
The V1 receiver, only 850 mA before the protection kicks in, so not enough to power 1 buggy motor, let alone 2.
The V2 receiver, 1.5 A before the protection kicks in, enough to power 1 buggy motor...

What might be a solution is having the V1 receiver control a medium motor or servo which mechanically switches a 8869 control switch which in turn switches the buggys on and off...

As for the old battery box, I think the reason you don't get it to power a V1 receiver is because power functions have two pairs of wires, a control pair and a pair that gives a constant voltage.

M,L and XL motors only use the control pairs of wires.

A servo uses the voltage pair to keep its position and the control pair to determine the position.

A receiver uses the input voltage pair to power itself, ignores the input control pair, forwards the voltage pair to the output voltage pairs and places control voltage on the output control pairs as it's instructed by the remote.

When using PF to 9V conversion cables, I think only the control pairs get connected, never the voltage pairs so the receivers cannot be powered by 9V battery boxes or cables...