...but it's not a dead ball.
Rule variation 4: During a try after touchdown the ball becomes dead when Team B gains possession or it is obvious that a kick is unsuccessful.
I'm watching game film and we had a R who killed it. I would imagine others would make this mistake as well, particularly given that the mechanic is to whistle the ball dead as soon as it is away or blocked.
Like you said, it's a live ball until/unless B recovers, or A legally scores (exchange then run, or pass).
For those that have not read everything above, this is a discussion of UIL 6-player football (and does not apply to UIL 11-player football).Sadly, there are many who follow the urban myth of sounding a whistle the instant the ball is kicked. Then, they get talked into making a roughing the kicker foul a dead-ball foul. No! No! No! That's why you wait until the ball flies past the goal posts, hits the ground, or is caught by B. But, as you note, if there is no actual legal kick, then the ball is just loose from a backward pass (in the example given), and may be recovered by anybody, and, if recovered by a player on his feet, advanced. That could apply to a fumble by the holder, as well (with no actual legal kick).
Sounding a whistle immediately when a legal kick is made is NOT an approved mechanic. Check Crew of 5, Section 26, Article 4, FIELD GOAL OR TRY, paragraph B, sentence 2. We are to wait, as noted there.