ah, hit the wrong button. anyway, question has been answered...

3. Yes, it is possible to book one-way rewards on AF, KL and NW flights only for 50% of the rt miles
If you are elite with FB, you'll have access to one more reward inventory (economy only), not sure with OnePass. But, with FB you pay fuel surcharges on reward flights (even for partners' flights where partners do not charge fuel surcharge) and with OnePass you don't
FB customer service is notorious

thanks for your input, Neuromancer.
so, based on your opinion, I should use the CO OnePass account?

are the FB 'fuel surcharges/ticket fees' like Lufthansa? LH charges a lot of fees for award tickets, in the end it comes up to a few hundred EURs. on UA, the taxes are very reasonable.

Opinions, anyone else?

Well, I suppose the final decision must be based on how easy it is to get an award ticket with both programs, and which airline you will fly the most, as some benefits (like upgrades, one-ways, probably award seats availability) are better if you fly the airline whose program you are member of. I do not know about OnePass, availability in FB for my trips is ok, if I am a bit flexible (I do not fly North America - Europe though). NW has a bit closer relationships with KLM (and FB) than with the rest of the Skyteam, so if you fly mostly NW, FB is a plus. FB also has these WebAwards, where on selected destinations you can fly just with 50% of the miles required.

The taxes on FB will be something like Lufthansa's, yes. For short trips comes about 50% of the price or the paid ticket. Flying from North America to Europe, with luck and careful planning you can get quite cheap tickets, so the taxes can seem high too. The problem is that even if you fly CO metal and pay with FB miles, you will pay ticket surcharges to FB just as if you flew AF. There is also the problem that an FB award ticket in Business is 90000 miles, in First 100000, a very small difference, but availability in First is low.
For me personally, if I were based in North America, I would choose a north american airline.

nope. no one-way flights on NW. KL and AF only.
Taxes are the major issue here and Elites do not get any additional C-class inventory on AF (although PEs do get this on KL (unpublished)).
If you are really only looking at award flights and the F option I would go with FB.

I have no problem with F-awards on AF, often more A than O availability

Award availability on CO flights is better as a Plat than Silver/Gold. You will have access to inventory that is only available to Plats. No one way awards and you can't redeem for AF first class.

One other thing to consider is that if you fly CO premium classes and are based out of Europe you will get 200% base miles, 200% Elite Qualification Miles and 2 Elite Qualification Points on TATL flights in J, D, Z or R.