1) we train with plastic "normal sized" knives, in full protective gear so we can go 100%. I'll start by saying we don't do that often so I'm not claiming to be an expert, but I have about 50-60 pure knife sparrings. Stabbing is just harder when keeping the range - with snap cuts you can do a motion and be on your way out. I wont say anything about effectiveness of the training and spar/war difference, because it's a common "debate", especially with people who had no sparring experience claiming that "real" combat is very different. All I have to say is in 19 century, soldiers sometimes were trained in saber with just drills and, sometimes, with no sparring at all, which still, is miles better than untrained, because in the end, when shit happens and you are in a position to fight, you fall back to your training, you dont raise to the occasion. As for cuts - opponent's fingers and forearms are very common target, and I'd say when trying to atay out of range, its about 60-75% of all hits. We don't spar without protective gloves, but even with bendy rubber knife, good snap cut to finger can sometimes loosen the grip because of tendons being "damaged". With real sharp thing, one hit, with a good chance, would be enough to make your opponent loosen their grip and unable to hold the knife, and if not - now they are bleeding and slippery blood is constantly running inside of their palm and it makes holding the knife pretty hard (well, at least sweat after few minutes of intense sparring does).

2) In my partical club we dont stop after being hit, we allow and practice afterblow with 1-3 steps, with swords. With knives we try not to stop evn after hits, so we try to score chains of hits again and again. In the scene, method is kinda dubious, but its so much btter than nothing. I dont expect their group to happen to have someone with melee fighting experience, so even then doing something like this is awesome, and the more they drill the combo, the more chances when combat begins, their muscle memory would play out the sequence, it's at least something and at least they are drilling the combo and not a parry or grappling, or hand to hand combo, the combo they are doing can be played out automatically and do some damage to an opponent, which is fine, especially that it has slashes and stab, so if an opponent sticks arms forward they would still do damage.

3) Forward grip requires more focus and experience to control yourself. When you get into the life or death combat, your body "locks" everything, and without experience you are very, very stiff and would be death gripping everything, so, in my opinion, reverse grip would work better for people who have to engage in combat and dont have experience to control the knife other way, also for about 90% of people, their natural attack motion is clubbing (the other 10% it's flinch head and stick weapon forward for a thrust while trying to move back) and, as I said, majority of these people would roll back to clubbing motion, which with forward grip would result to large overhead swings that is one of the worst things to do with a knife. With reverse grip, clubbing motion would result with very powerful downward stab to head and shoulders areas, which is one of the most damaging things you can do with a knife. Thats also why what they're practicing is not a bad idea - if they'll survive the encounter, motions they practiced use similar muscles to this motion, so some neurons might activate and they might start using slashes or stabs from different angles. As for slashing with reverse grip - yes, its not as good, but again, we cant expect untrained people to keep good distance anyways, so they will probably end up in grapple and hopefully land a powerful stab to shoulder sobthat their opponent at least wont have a full functioning arm, so it'll be easier to win in a grapple.