That's clearly not what it's trying to do right now. For one the UK is still a member state and still paying its dues, but the EU is acting in an extremely hostile way. Unless you reflexively define the best interests of the EU Commission and related institutions as "the best interests of all and every single member state", it is certainly not the case.

Consider: The EU is setting things up such that in just over a year trucks will start to pile up at ports, costs will rise across Europe and the UK, many European firms will be cut off from the financing they receive from London, airplanes will be grounded, Irish politics will destabilise due to the introduction of a hard border and that's really just the beginning. None of this is in the interests of any person in any EU member state at all. And it can all be easily avoided.

Your spa analogy is very badly off by the way. Trade isn't something countries sell like a product itself, despite the EU's attempts to package it that way. Countries that strike free trade deals do not "buy" them, they negotiate and agree to them. Think about neighbours cooperating more than a consumer buying a product.

No, it isn't. Spa membership doesn't dictate what you cannot do outside the spa. Spas do not collude other competing spas to exclude you from interacting with them.

Second, some privileges are real, and some manufactured to punish outsiders. Prioritization and restrictions on trading with non-EU is an example of this, which addresses the "hypocrisy of protectionism" point.

Finally, "because that fits their needs and suits their best interests" who's interest? Brexit voters? They got one vote premised on an EU promising the moon, and little control or feedback sibce then until the recent Brexit vote.