Discontinued products don't worry me too much, on the cost of changes v benefits from optimising they rank pretty low to me.

I just checked how my site works if I plug an out of stock product in to the URL & it shows "product not found". I like your idea of showing the product page with "out of stock", but don't know how you would do it.

The idea of marking each product with a different type of "out of stock" marker sounds like a lot of work though. For my stock (no seasonals) I would want to mark them with "contact us to see when it is back in stock"

A question for you - why do you have products you don't sell in x season? I can understand in a bricks & mortar store, where you are constrained by your space, but online you are probably not. Why not just sell the products all year? Sales of fans will be down in winter, but there will probably be someone.

I think the URL showing out of stock items is a good idea - any clue on how to make it happen?