The bushed in the pivot are very widely spaces, so if they are in good shape, you should have a pretty stiff rear end. Even today, nothing spaces this wide apart. They can take skin off your ankle bone though. The bushes are cheap enough and easily sourced- hey are a stock part.
The top "link" is actually not quite a motorcycle link, but a link from a production machinery drive chain. With a new link PROPERLY installed (press tool to compress the deformable pins), you should have again a pretty tight rear end.
Stock shock has a very large bushing area which again helps support everything. Very simple coil spring, oil damped. Wind the shock eye to adjust the preload (sag), and change the oil weight slightly to change the damping. 5w is stock. If you're brave you can adjust the valving. Only down side is it's a heavy shock, BUT you're talking about a 4130 steel frame full susser so it's never going to be flyweight.
Yup, Risse still supply an air shock that saves IIRC 3/4 lb, but the damping holes are oo small as stock giving a very lethargic shock. This is intentional. You need to drill and modify these to tune the shock to your weight and style. Not a quick exercise, but it is easy to strip.
This is an old design. Gazelle is 1 1/4" head tube and may or may not have a cranked seat boom. Defiant should be 1 1/8". These are old skool machines, so forget lobbing on even a judy, let alone 100mm forks. Stick to period M1/2/3/4 or mag 20/21 (Earliest gazelles came with colour matched RS1, but even these could feel a little long) and period stem length and bar width and it will handle fine. Try and make it modern and it will feel like a chopper...
Gazelle- with square section tube by the BB, you may need a suntour front mech to clear a 46/48t ring (clam is lower than on M730). Defiant, shimano mech should work fine.