I am not an expert but I've had a '53 Vac 13 for 12 yrs and done some research here and elsewhere. Hopefully someone will correct if I am wrong.
1, Yes 1955, '55 model year starting serial nr is 6038001 for all VA series tractors.
2. No. VAI had some exceptions to other models styling in later years but remained model VAI.
3. I don't know top speed but it is way more than other models. Full governed engine rpm for VAI is 1925, other VA models are 1600. Some VAI had a speed blocking lever (pic) to reduce the rpm for pto work.
Yes they have a higher speed transmission. Some VAI & VAC had higher speed transmissions for mower work (highway type). Gears listed in transmission section of Case part manual R.I E61, dtd 5-59.
I have the mower transmission in my VAC. When I bought it, drove it home 8 mi on paved roads. Very fast. I am not going to tell you what my son following clocked me, did that a few years ago and got flamed. when I got home and changed my shorts, checked the rpm, 2200 rpm no load when I backed off.
4. My VAC left to right is 1 up, 3 dn, 2 up, 4 dn, with rev all the way left & dn. I drove a '46 VAI in the '50's 1-1/4 mi, red dog road, twice daily, 6 days a week with milk cans for several years. I seem to remember it was a different pattern but don't remember what it was.