mosfet Ov protection
No, the capacitors across the MOSFETs will not do much, I do not think it is usefull to include them.

I think you're being overconcerned with protecting your MOSFETs. The most sensitive part of the MOSFETs is their gate. These MOSFETs already have build-in gate protection. You included resistors in series with the gates (good!) but these are only 10 ohms. I would make them 1k ohm so that no damaging current can flow.

You have to ask yourself what you want to protect against specifically. Just "voltage spikes and transients" is too vague. Do you want to protect against the 12V wall adapter failing ? Although it failing and supplying too much voltage is a very small chance you could protect against that with a fuse at the input and a 15 V zener diode across the supply after the fuse. When the supply goes above 15 V, the zener will conduct and blow the fuse interrupting the current.

I do not see a reason to include more protection, that just seems paranoid.