On the Maple, we put the end sealer on the same day as we stacked the blanks in the drying shed with stickers.
Do not leave your slabs in the sun, get them under cover as soon as possible. Don't drape a tarp over the slabs, this just holds in moisture, you need a lot of circulation and air space between the slabs.
Air cure is best for gunstock blanks, heat will weaken the internal fibers. With softwoods I do not think you will have as much of a problem, but I would not use a conventional kiln, fast drying causes cracking. A warm dry shed with circulation should be OK. We had dehumidifiers in the drying sheds.
The teak oil takes many coats as it is oil and soaks into the wood. The more coats the better it looks. Spar varnish will not take long as it stays on the surface of the wood.
Buy a moisture meter with a set of the probes that you hammer into the wood. This one has the contact probes that you push into the wood and I also have a set of the hammer in probes. Bought this one on Amazon.
11 or 12 per cent is about as low as the maple will dry to in my area when air curing, a long hot summer will bring it down little more. I bought a walnut blank from Idaho that was air cured 3 years, it was at 4 per cent when I got it, Southern Idaho is hot and dry in the summer!
Try some sample pieces and see how you like the finish. Spar varnish is made for outdoor use, originally made for the masts and spars on sail boats. Using pre stain before applying spar varnish gives a darker, richer color.