Is it possible to use the “Go to Page” action to send the user to a Suitelet AND pass parameters to that suitelet?

Probably not - but I wouldn't completely rule it out

What’s your go to method for an approval workflow that requires the user to provide a reason if the approval is rejected?

I don't have an answer there offhand unfortunately.

I usually use a suitelet with a textbox to enter the reason. I use a custom workflow action to call the suitelet where they enter the reason