water stains on beech wood strip flooring

I moved into a house where the previous owners had installed beech wood strip flooring throughout the ground floor. The beech is oil finished (matt).
I have a few questions which I would be grateful if somebody could advise me on:
1) Is it possible to remove black stains caused by water soaking into the wood? 2) Theres a nasty stain caused by dripping oven cleaner in the kitchen! Is this beyond repair? 3) Can the flooring be treated with varnish/wax or something to keep water out -I would only want to do this in the kitchen and hall which is most at risk of wear and water 43) Any advice on cleaning, improving the wood. Buffing? Mopping? Rubbing in oil?
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