Sounds like a decent plan, but I would put down a preemergent now as it will help prevent crabgrass from popping up in the upcoming months. I would also start attacking the wild violets and clover asap, as those typically take multiple applications to knock out fully, especially violets. Doing a few spot treatments throughout the season will also make life easier when it gets closer to seeding time...

1) Yes, you can use 2,4d and Quinclorac at the same time if you want. Many box store and pro mixes contain both.

2) Feel free to use both as they attack different weeds and work well together.

3) Various laws & regulations. May also be a Amazon/ Southern Ag decision due to labeling or shipping issues.

4) Since you mentioned wild violet I would recommend T-Zone (if you can find it, may have to try different sellers/ stores) over SpeedZone which may be more cost effective. As it already contains Triclopyr along with conventional broadleaf herbicides 2,4d and Dicamba. It also has Sulfentrazone which can help with any sedges and will likely have effectiveness on violets.

https://www.amazon.com/T-Zone-Turf-Herbicide-1-Quart/dp/B00NWSIHCK

https://www.seedranch.com/TZone-SE-Herbicide-1-Qt-p/tzone-se-herbicide.htm (Using a sample address it looks like they ship to Mass.)

Be sure to read the labels! It might help to check out https://www.lawndork.com/ to help find the proper use rates and break down of product labels into an easy to read format.