Ouch, ouch and ouch.... sorry and ouch (4 hits).
1. Really dependant on driving - this car will drink some if driven that way.
2. You should either teach yourself to do some stuff on your own, or at least change your mechanic. Your story is filled with unprofessional parts. If you bought a really cheap car and they were transparent on faults etc then it is of course your risk on doing so, but it sounds more like they have been covering some stuff here.
2b. Regarding buying parts and bringing to a mechanic i'd say it would be a matter of their business model. If their margin is within the work or parts, it should be a bit of both. But it should be reasonable for all parties, win-win as always, and if it aint you should debate it.
3. Now you REALLY need to change your mechanic. If they dont know about the IMS it's no discussion about it. They might know stuff about cars, but not nearly enough about Porsche to be working on your baby. The IMS itself is a bearing that have had numerous debates and discussions due to it's habit of braking in X% of the case.
IMS readup for you:
https://www.oregonpca.org/wp-content/uploads/2014/06/ORPCA-IMS.pdf
Best of luck!