No idea about the seat switch (maybe a parts diagram for the mower will give you a hint how it comes apart). Maybe test what the voltage across the battery terminals are when the PTO is engaged (may be non-trivial to do), as it could be:
-bad battery (even if it's new, it can be bad)
-bad electric clutch (uses too much power)
-bad charging circuit (can't put out full power, perhaps has wrong parts on it, or parts have failed or installed incorrectly)
-bad battery (even if it's new, it can be bad)
-bad electric clutch (uses too much power)
-bad charging circuit (can't put out full power, perhaps has wrong parts on it, or parts have failed or installed incorrectly)