1. Wasn't the "jack test" just a standard thing you'd do when buying a used Beetle? Seems to me any boasting about a Speedster passing the jack test would be related to the proposition that "this here old Beetle pan may be cut & welded but it's solid enough to pass the test you do with an unmolested original donor car."
2. Spyders should handle better than swing axle Speedsters because the weight is better distributed and the torsion arms are longer. A well set up IRS Speedster should be able to out-handle a Spyder because IRS is all that. I am currently unaware of any skid pad results or other data to back this claim up, however.
3. Yes it is technically possible to use the Subie transaxle in a Spyder (without flipping the R&P) but it will be a fettle because the chassis crossmember that mounts the rear of the tranny is not set up for the Suburu rig so you'll have to rebuild that, and also the Subie is an independent suspension system so you'll have to either convert it to a swing axle type or engineer an IRS system into your Spyder chassis. It's all quite doable (and has been done, I believe) but not a task for the beginner.
4. Yes, Subie transaxles can be used in a Speedster (if you flip the R&P which is spendy and tricky) but also you'll be modding the guts of the car a lil to make it all fit.
5. The fetish for a 5 (or more) speed is very strong for some reason and it's really quite silly, on a dollar-per-grins basis, unless you're the type who just has to run 10/10ths all the time & so are equipped with a high-strung engine with a little teeny tiny powerband way up the der stratosphere. Basically any good street engine (and especially any Subie) will be "on cam" at least from 3500 to 6000 and it really isn't hard or expensive to build a VW box that will handle the torque you're making, land that engine in its sweet spot between gears, be able to hold a hill at the stop light and cruise at 70mph+ without deafening you. With a Subie 5 speed you're adding weight and complexity galore and spending cubic dineros to do it & in the end, yeah, sure, it really is "better" but you better really really want it.
6. +1 on plug welding. It gives sad-sack duffers like me a little "cup" to spin the wire into and that makes all the difference in getting both good penetration and smooth(ish) beads.