Agree with the above answers, although I haven't tried the Herbie's Mat with the Simplex. I have used one of those mats with my previous tables with excellent results. Here's some more info:
6. Can anyone recommend footers that are a significant upgrade over the Well Tempered’s stock feet?
The Herbies Tall Soft Tenderfeet work as well if not better than the stock squash balls, without problems with the table maintaining level due to the unequal wear on the squash balls. I've also used Herbie's Turntable footers as well, but I actually think the Tenderfeet might be a touch better in my setup, which needs isolation from footfalls.
7. Can anyone recommend an aftermarket dustcover for the Well Tempered turntables?
Local plastics suppliers can fabricate a custom cover, but you can also DIY. I measured the size needed for my Simplex, and raw materials would cost about $60 for custom cut acrylic of 1/4" or 1/8" sheets. Acrylic is about the easiest material to work with: cut, sand, glue, hold a few minutes while it sets, sand again, clean, then it's beer-thirty.