1. If you are a developer, get a macbook. Anything you can do on linux can be done on macbook + more.
2. Laptop vs desktop, unless you are on a very tight budget and mac is essential for you workflow (like iOS development) then get a mac mini. If a mac is not essential and you are on a budget, Then get a good windows laptop, You can buy a good windows laptop for the price of mac mini (But I don't think you can buy a Thinkpad for that price)
3. No. in some cases, buying in LK is cheaper (No idea how those sellers are doing it), If you can get the student discount + VAT refund, then buy from any country. I think you can extend the apple care online after the first boot.(Never done tho)
4. lot of places offer installment. (LuxaryX, Francium, Abans etc), but the total price would be higher if you do so.
I don't think you have much upgradability in modern Thinkpad. Most of the new Thinkpad comes with soldered CPU + Soldered RAM, and some has soldered SSD (there are some models with single upgradable RAM slot + NVMe slot). So this is not a significant factor now.