Yes, it is normal, not only in Greek Islands but also in Greek cities, especially in high-end places.
Unfortunately, the whole point is usually to not appear to officially work for the bar.
Is the valet properly insured and officially employed?
Does the establishment hold a valid license for the parking space?
If there is a fee for parking there (...
September is not exactly 'post-season'. It's surely not gonna be like July and August but there are gonna be some parties till mid September and the island is not gonna be empty.
Here's a blog where they write about events in mykonos.
I've seen tourists kayaking without guides. I don't think it's a common practice that guides should be obligatory. I think you can work things out with a company and rent some kayaks without guides.
Honestly speaking, all the islands are great for exploring.
I've seen that you can rent a kayak in Zakynthos that is really exciting and has some great spots to ...