Disruptive Frontline Gameplay

[Scintilla]

It seems likely (although unproven) that on all three maps, optimal strategy is to build BH first, then use that to make capturing objectives easier.

That is the plan: build BH early on and objectives will be far easier to win and hold later on.

Indeed when I point out that you don't get any bonus points for capturing Mid on Onsal, I often get shut down by the assertion our team is there to build BH.

This one, in my opinion, isn't as straightforward. Yes, in the case of Onsal, the middle zone is probably the easiest place to farm BH because everyone is gathered in a single area, with each entry point being a bottleneck ladder in which easy-kill casters and ranged group up. Who needs a voke when they're all gathered together for you! Drop AoEs and LBs on them and watch the kills/assists climb. However (assuming you do the classic approach and enter the platform by your own ladder, rather than circling around a team to attack them from behind) the same layout limitations apply to your team also, so it can easily be just as unforgiving as it can be beneficial.
Premades will still continue to win here, not so much because of the DRK voke. That will still help in the holding of enemies to prevent them running, but the primary advantage here is the coordination. In a random team, few will call their LBs. Most will be using their LBs/AoEs as and when they're ready, spreading the damage and making it easier to run/heal/guard through. In a premade, the delivery will be simultaneous - a quick attack and withdraw.
This is somewhat a fault on the part of random teams playing more as an individual rather than a team. That said, even in they event they did seek to coordinate, they would still struggle to reach the same level of coordination of some premades - though the difference between them would definitely be smaller.

The issue which I have with that middle platform is how blind to all else that people can become.
In a premade team, substantial points can be farmed from kills alone and objectives can become almost secondary: a 'bonus' which comes as part of having high BH. But in a random team, the chances of farming substantial points through kills alone isn't remotely as high, so there is a little more reliance on getting objectives to boost those points (and lucky RNG to make getting those objectives easier, as they likely won't have the BH to do it). If a team is blindly focusing on the middle, not doing a very good job at it, and have been there for a long time already, they'll likely have given up a lot of other surrounding nodes to other teams with minimal opposition. If it's late in the game, that might be the difference between a win and a loss.

The obvious weakness in this response is that it presupposes simply being on Mid magically creates BH. It doesn't. The team it strongly favors is the one with a DRK+ premade. My guess is that if you find yourself on a random team, particularly one without a couple of DRKs, the best path to winning may be to hope RNG runs in your favor and focus heavily on objectives. Certainly worth trying an early flanking attack, partly to see if the other non-premade alliance is on the same page, but with the premade having an expectation of winning 70%+ of the time on average, it may be the best way of increasing your non-premade team's odds above 15% is to cap nodes and hope for favorable spawns.

The big problem is premades cannot be avoided. While some will call for the team to focus entirely on objectives and try to avoid all PvP with the premade, that won't happen. They'll come whether your team want it or not. And when they do, if the team has avoided PvP to that point, they won't have any BH to have a hope of defending themselves. As it stands, the only options I see are:
1. Make a premade of your own
2. Hope that you have a few experienced players who are able to make use of WAR/RPR/MNK/WHM etc. to counter them
3. Take every opportunity you can to pincer them while they're launching an attack on the other alliance. They'll have used their burst and they'll be focused on the other team. Now is the best chance you're likely to get (but I admit, getting a random team to do this is far easier said than done)