First, I hate to have a defender A and an inbounder B - just not right! Please put the players in their proper roles
This rule exists to potect the offense from overaggressive defenders reaching over the line and preventing the inbounds pass from being made, or causing a bad inbounds to be made. I would come down on the side of a no-call if the toe on the line
clearly did not disadvantage B. RecRef, consider a situation where B1 totally fakes out A1 and turns to pass to wide open B2. Where has A taken unfair advantage? Sounds to me like they got fooled and B has an advantage. I would say this especially as the coach of B. In 8th grade boys, you have only so many inbounds plays. You may disadvantage B if they ran a good play (which A has now seen) and you make the toe on the line call. I have some special situational plays that will only work once, and I save them for a close game when I really need a score badly. They will only work once against a good team!
Complicating your life after no-call, A ends up with ball in your game. But if you felt at the time the lunge was made B was not disadvantaged in any way, I would think you still should not make this call. And I can't tell you how you know for sure, you just gotta go with what you see and live with it. But given that A1 subsequently tips the ball, I would be sure that the tip was clean and did not involve an early reach past the plane - and be ready to call it if A1 did violate to make that touch. Also, if B hesitated while A1 was on the line, A1 gained advantage (which may have helped him tip the ball subsequently) and the call must be made. In that case, B needs to be able to reset and have a new full 5 seconds to inbound.
Of course, in this situation you did not think A had gained an advantage so you decide to let B play on - not necessarily the wrong call, but it turns out looking worse with A stealing than if B inbounds to wide-open B2 for a lay-up. Just reflect and try to visualize the situation and make sure in your mind that there was no advantage gained on the initial step on the line. If you feel it was not, I think you made a good no call and B's turnover was in no way related. If you think that A actually may have gained an advantage, you now have a new perspective on judging advantage-disadvantage so you can make the correct call next time.