Q1. Probably not. You can buy bedding plants ready to flower as early as you could possibly put them in the garden and before.
Q2. Makes sense if you need a lot of plants on a small budget or if you are very particular about varieties and colours which may not be easy to find.
Q3. To some extent this depends on the plants. I mostly go with the six-pack (1206) size but some bigger plants are definitely rootbound in these. Go with whatever size will give you the number of plants you need from the space you have, and don't sow too early. You will probably only need to grow under lights for about a month, then harden off to outdoor conditions for a few weeks, then plant out. A few "annuals" such as Impatiens are a little slower.
Q4. Use one T5 tube to cover about 6" of width of plants, 4' long.
Q5. 4 1'x2' flats will work nicely under a 4 bulb fixture, but watch out for the plants at the edge leaning in because the four tube fitting is less than 2' wide and won't fully light the edges. You may need to turn the trays occasionally.
Q6. I think you only have a choice of 6500K and 3000K bulbs, you can use 4x6500K or two of each, I don't think you will notice the difference. Don't use just the 3000K on their own.