You don't need a place to slide back the lumber. Those two metal strips on it's top is made for that. Find a used kitchen cabinet that has a drawer or two and mount it on that. Put locking casters on the cabinet and your good to go!
It's just a home garage shop... very beginner-level tools. I'm making a tenoning jig for my table saw and a mortising jig for my plunge router this weekend. I had planned on a single tenon but I saw some things online about double tenons and wondered if it would really make that much difference.
I'm certainly not an expert, but it doesn't sound like you'll be stressing your joints that much. Your table will be holding what, 150lbs at the most? Think of a step stool that needs to hold 200+lbs, or a two-person bench. Neither require (though some might feature) a double m&t. What about using a drawbored tenon? That gives you physical strength in addition to the glue.
I don't know what your particular shop setup is. If you're using a Domino, there's almost no additional work involved, so no reason not to add more tenons. If you're doing it by hand, a single tenon is going to be a lot more accurate.
PNP/Drywall mud anything like that is not strong enough. As, you mentioned will turn transparent. A good grain filler I like is from Woodwise. Goes on nice, sands easy and water cleanup. The wood floor industry use the product and comes in many colors.