We would be cautious about wax or oil finishes for a handrail. Such finishes are inclined to become grubby with regular handling, particularly if you are considering a handrail for a commercial building. We suggest a satin grade floor varnish. That will bring out the natural colour of the wood and provide a hard-wearing finish that can be wiped clean when necessary.

We weren't sure how the plywood is supported. If it's laid on a concrete slab and then battened to attach the floorboards, there should be a vapour barrier under the plywood. In that case the limited airspace shouldn't be a problem since the floor system will be protected against moisture from the slab. So nailing the boards would be adequate without gluing, as long as the battens are deep enough to accommodate the nails. If the battens are not deep enough to accommodate adequate nails, then gluing in addition to nailing may be necessary.