"what in the? excatly, what was the purpose of this "frankenplane"? and....why? why create this...abomination?"
It was supposed to have exceptional performance and long range. The P-75 failed to meet expectations and was cancelled after roughly a half dozen of them were built.
If you think the P-75 looks ugly, look up stuff like the XP-55. The USAAF focusing so much effort on so many failed designs is partly why actual successful programs like the P-51 were delayed so long.
J_A_B