If the manufacturer recommends #8 there is most likely good reason. Motors frequently are not happy with the voltage drop that smaller conductors over longer distances will bring. #12 and a 20A circuit is marginal for something that claims to draw 19A. I can almost guarantee you're going to trip the breaker from time to time. What circuit rating does the manufacturer recommend?

No you CAN NOT parallel #12.
No you can not "unravel" the #8. If the plug or terminal won't accept wire that large, you're going to have to make a connection using some listed means (they do make wire nuts that large, for example) to join to a wire that fits.