The black stains are caused by mild steel staples which were used during manufacture to hold the second layer of veneer while the third layer was added. As a result these marks are usually near the centre line or at the joint between components. You will be able to partly remove the staining but it will always come back unless the staples are removed from the lower layer. However there may not be much of them left by now!
The oxalic acid is the correct way of minimising the staining. I have used bleach in the past. Either way it needs washing well and drying afterwards. Otherwise just live with the staining – it is part of the character of the boat!
As far as finishing afterwards. I would use a clear varnish. My coachroof is coated in traditional varnish. Six coats, and an extra coat every year. It is much easier to renew the surface yearly – to retain the UV resistance – than to let it deteriorate and have to strip it back again. Incidantally try not to sand too much off – or you will go through to the middle layer of veneer. You certainly dont want to have to strip it back too often.
I have used boiled liniseed oil as a primer coat in the past. Then overcoat after a few days with normal varnish.
If you want to use epoxy. You could roll on two or three layers of epoxy to soak into the wood before applying 2 component epoxy on top. You still need lots of coats (6?).
I have been reluctant to use a glass cloth with the epoxy because I have had trouble getting all of the air bubles out from under the glass cloth. Others have used it with varying amounts of sucess.