1. When priming wheels(pre-made - unsure of material - possibly pine? sold by craftparts) and mdf toys, how long do people wait before painting with acrylics or sprays?
It depends on conditions. The higher the temperature, the faster the shellac dries. The lower the humidity, the quicker it dries. Shellac Dries very fast. I have used shellac for several toys at once where I could work continuously. That applies a coat of shellac to all the toys, sand and recoat without having to wait.
Store bought wheels are generally mostly end grain. They will soak up a lot of shellac and take longer to dry. I've had shellac go all the way through wheels so I would leave them overnight.
I believe the wheels from craftparts.com are birch.
2. Is it ok to use only one coat of shellac?
Yes. I made a tray puzzle yesterday. I applied one coat of shellac and knocked the fuzzies off with some 240 grit sandpaper so that it felt smooth to the touch.
Apply shellac until it looks like you want it too.
3. When painting with Red acrylic it still appears blotchy after 5-6 coats? Is this because I used white Gesso and should have tinted it before applying to mdf edges? or am I not using a thick enough coat of acrylic?
Some acrylics do not cover well. I trashed a can of bright green paint because the first coat I applied was transparent.
I don't use gesso.
I generally don't use primers or sealers. I apply the paint directly to the wood or MDF using white paint if the end color is going to be yellow or similar color. I don't think primers are necessary in most cases. If you are using pine with knots in it you might need it to seal the knots.
Multiple thin coats of paint will give you better results than one heavy coat.
4. Do acyrlic paint qualities vary from very bad to very good or much of a muchness?
I don't understand muchness?
Same brand same base but different color can give you different results. As an extream example, black might give you one coat coverage while yellow will need many coats and a white undercoat or primer for best results.
5. I am also going to try priming with shellac to see if it reduces the blotchiness and showing through of white Gesso. Is this a better option?
I don't use gesso.
Shellac will seal almost anything, but it isn't a primer. There is a shellac based primer. Try just using some white acrylic paint. It will seal the MDF and provide an excellent base for your color coat.