It isn't. The front double wishbone setup is consistent, but the rear changed significantly. Mk1 used a swing axle; Mk2 and early Mk3 (to KE/KF20000) used Rotoflex independent rear suspension; late Mk3 (from KE/KF20001) switched to a swing-spring layout. Confirm your chassis number before ordering rear suspension parts.
They are. Rimmer Bros stocks Rotoflex couplings for Mk2 and early Mk3 GT6s. They're listed on the GT6 drive shaft page alongside related driveshaft components. Inspect them for cracking as part of any suspension service.
Yes, and worse. Neglected front trunnions that aren't lubricated with EP90 gear oil will wear progressively and can cause the front suspension to collapse. It's one of the GT6's most important maintenance points. Front trunnion and vertical link parts are listed on this page.
They can. Changing rear suspension geometry, particularly on swing-spring cars, has a knock-on effect on rear axle loading. The GT6 Performance Upgrade Parts section covers polyurethane bush kits and handling upgrades designed specifically for the GT6.