Is a heim joint a bushing?               

A heim joint is not a bushing . A bushing is a device with an inner sleeve and an outer sleeve connected by
some type of compliant material. A heim joint or rod end or spherical bearing is a precision steel or alloy
body fitted with a precision steel or bronze insert.  This type of precision bearing has no place in stock or prepared class racing as a replacement for a bushing.

Please note these two exemptions:

Stock class: sway bar end links often are comprised of two rod ends linked together 

Prepared class: most adjustable front upper camber plates incorporate a -08 or -10 spherical bearing to replace the rubber upper mount(bushing)

May the rear seat be removed if the car is still within weight?

Seat in rear can only be removed if roll cage design makes this necessary.

May 'camber plates' be used in the stock classes?  

A fixed camber plate that fits between the unaltered strut assembly and the unaltered body and provides no adjustment is allowed.  An upper strut mount assemble is not a camber plate. In the stock class the car must be equipped with the factory original upper front strut mount. It must be used without modification to the part. The body holes locating and securing the mount must be UN-altered and used in the original configuration to secure the mounting. No slotting, modiification,or substitutes are permitted.