Hey Miguel! Yes, the rear 'fin' is to provide support to the rear wall. However, this shelter, because of it's relatively high profile (lots of headroom for me at 6'4") and large surfaces, would not be a good choice for a trip where you expect heavy exposure to high winds and inability to find somewhat sheltered spots. Everything is a tradeoff. We have not seen problems with the fin catching wind, it's a pretty small surface compared to the rest of the tent.
There is no pole in the ridge. There are two screened openings in the rear wall for ventilation, which may or may not be necessary. On this prototype there are 'flaps' that can be used to close the openings from the inside, these are definitely not necessary and will be dropped.
I have no idea on the $600, maybe it would 'only' be $400… we're simply not at that point yet. We don't anticipate making too many of the cuben fiber version.
Glad to hear you're still working on your shelter…don't make it TOO small!