Undecidability of the language of PDAs that accept some ww
Your proposed idea doesn't work, because after you have checked the first half of the input with your PDA, you have popped them all so you cannot reaccess them for comparing with the second half.
The correct idea should be making a PDA that checks if odd number configurations leads to even numbered configurations in the first half and vice versa in the second half. Also, make sure the PDA handles the start and accepting state. Then convert the PDA to CFG and pass it to the decider for the language $\{ww\}$. Accepts if it accept and rejects otherwise.
This will work as you can show that the PDA as constructed above will recognize string of form $ww$ iff $M$ accepts $w$. This is because the parity checks of configurations only gives incomplete information, but when supplied with the fact that you have two equal computation history in some string, it tells you each half of that string is actually an accepting computation history of $M$ on $w$.