The wood hasn't fully dried and it will continue to move until it hits about 6%. Also, one piece sitting on stickers won't necessarily dry flat- put some weight on it to keep it flat or it will continue to move. You could use a piece under the slab, oriented at a right angle- this is sometimes called a 'strongback' and when it's fastened, it causes the slab to become straight.
I have used Minwax poly many times and have come to the conclusion that their floor poly is kind of an unknown finish for furniture. It dries quickly and is more durable than the regular stuff. I have done some test pieces and one time, I accidentally dropped a piece of green ScotchBrite in the pan and since it was scrap, I decided to scrub it into the wood. It did a few things- continued to smooth the wood, scrubbed the finish into the surface and anything that was removed from the wood acted as filler. It dried faster that way, too- super smooth. Once it was fully dry, I used a dry piece of white ScotchBrite to burnish the finish and it was left very smooth with little marking from the abrasive.
If you plan to use paste wax on this, I recommend using white ScotchBrite to apply that, too. I did that to teh face frames when I built my kitchen cabinets and the finish was smooth, with a soft glow- not glossy, not satin.
BTW- gloss finishes are the hardest because they don't have any modifiers so you can use gloss for the first coats and finish with satin if you want, but I would recommend using the ScotchBrite.
If you do use shellac (there's not really much reason to if you plan to follow with poly unless you need to cover sap or something else that can be dissolved by the chemicals in poly), you can pour it on and brush it or roll it. The finish doesn't care if you use traditional methods and whatever works, works. You can also use a cloth, piece of carpet, etc and if it's wet enough, it won't dry instantly. If you can do this when the temperature is lower, the drying time will be extended. If it isn't totally smooth after the first coats dry, it will be after subsequent coats.
I bought some books on finishing, one by Jeff Jewitt. Be creative- you can thin almost any finish, just use the correct thinner. I brushed the poly on the base cabinets and sprayed the upper ones after thinning with mineral spirits. I have also sprayed smaller pieces and it really speeds the process but it's not mandatory.
My mechanic told me "I can't fix your brakes, so I made your horn louder".