Arch: i dont tell people to try to initiate a loose pivot because 99% of players will not be able to hang on beyond the critical point.
I am like you in that i can initiate a loose pivot but time the grip pressure to tug the pivot harder that way. You are the exception and not the rule
i have worked with a few players that were failing at drills because of this.
The thing i have come to learn is that it is not the pivot as much as it is the raw "edge weight" (aka the weight of the hammer's head) that is really being forcibly manipulated here. This explains why it is still possible to snap without extending the wrist as long as the relative positions of the forearm line up with the body positions and timing.
As i have said with all of these things, im not preaching a fixed action, only teaching a particular feeling that is unque to snap-based throws. That feeling can be achieved in many different ways, none of which are wrong, some of which are easier to learn, some are more consistent, some are stronger, etc
Based upon the distances you listed, you are on the cusp. If your wing span is under 5'10" you are probably full hitting. If your wing span is 6'3" or higher, you are likely closer to a half hit.
Try getting the water to shift to the front of the bottle, it may help boost your d another 30' or so.
Sd: with the rail method, if you hold onto it beyond the critical point the disc leaves when one of 3 things happen:
1. Your speed dramatically slows down
2. Your arc breaks off its curve and abruptly changes shape (e.g. Goes from round to flat in front).
3. Your arc passes the primary frontal apex and begins moving backwards (aka full hit zone)
As for staying online or not, i only worry about the last 6-10" of throw being correct, as that is where power and accuracy are determined. I suppose i could probably put more work on that (i tend to only analyze it when it causes the later parts to fail) but that's the next step: working with students that have broken to the last plateau and have begun fine tuning. 99% of my lesson work is targeted at hitting it harder.