Tavaly tavasszal hol voltal and Tavaly hol voltal tavasszal is the same
Are tavaly and tavasszal related?
Their etymology isn't clear, but they aren't related.
I tried to look these up. I found this much: "tavaly" comes from "távol" (far away), and "tavasz" is of Finno-Ugric origin. It would be nice to know more. But they are not related, that much is clear.
Is the instrumental case used for time periods other than seasons?
Is "Hol voltál múlt tavasszal" wrong?
Yes, because "múlt" means "past", the last year is more specific (it can be also "múlt évben). :)
Can somebody tell me the difference between TAVASZ and TAVASSZAL?
Hol voltál mult tavasz? Should be accepted, but isn't.