Comments- The only Hyperglide item you might have is the chain. SunTour would be sued by Shimano if they used a trademarked term from Shimano.
- Yes the last two small cogs should be threaded on, some SunTour freewheels had only the three biggest cogs splined on so it can be a find out as you go to a degree.
- Yes chain whips can and do break before cogs loosen sometimes. A stronger whip and more leverage is the solution. However I see no need to remove the cogs but they are not mine...
- I have taken apart many of these and other freewheels back in the days. Usually the internals are in good condition as they should live in an oil coated world so shouldn't rust quickly. I agree with the others in that taking the internals apart is a job that has risks and little benefit compared to a flushing out with solvent and dripping in thick oil. If you do take this apart note the amount of slop between the inner core and outer half of the body. ST used very thin shims/washers under the hammered tight threaded race (note it's a LH thread). When reassembling the race can end up a bit less or more tight and thus the bearing "adjustment" can be off. It's usually suggested that the end result has a tiny bit of slop to the bearing adjustment. Having a spare FW for parts is a good idea. Don't use grease to relube, use oil.
- The only two times I've had a real problem in doing this stuff is when i failed to fully secure the race on reassembly. Then some miles later you hear a faint jingling of the race rattling about on the axle end just before the outer half of the body slides off the inner core enough for the balls and ratchet pawls to fall to the ground. It's no fun to turn your bike into an awkward scooter
Andy