In the past, I've seen reflex jump up to an opponent who was above him and use it, even if it was for a short burst. I've always been a fan of Nair -> BS and it's pretty reliable. From my last tournament, I learned that sometimes waiting for that moment when you notice your opponent tries to jump over you, punish them with BS. If you are certain your opponent will air dodge, then punish them when they land, for this in particular I have to say is really impulsive.
Tethering: I've had the same issues. Vine whip will reach super far and look like it's gonna grab the edge but doesn't. It'll be a judgement call for you when you should go for the ledge. I have the habit of throwing a razorleaf dead center at the ledge for insurance. Most good players will crawl back on stage with their invincibility frames right as you are about to vine whip them and it still causes a ledge hog. Sometimes it's best to go for the ledge immediately, others, like I said, you want to make sure you can actually get back, like using razor leaf. On less experienced players, at a certain percent and with the right aiming, you can spike them off the ledge and KO them before you get KO'd.
Squirtle shouldn't obviously go far beyond his recovery range (unless it's the last stock and you want to take that gamble) but it is notable, if you can pursue an opponent off stage and you are capable of doing it, then do it. Squirtle can't trip while hydroplaning but can trip while shell shifting and/or dashing. It happens to me occasionally and it's a little frustrating.
The first question I'm not sure. I might be misunderstanding your question. The second question, I think you can, I'm not entirely sure, I don't know if I've ever tried it. As for the glide attack, I find it useful. It rarely has ever failed me and you get a lagless land which leaves for an immediate follow-up
Hope this helps some.
