This is the mess we get into when trying to fly fixed-bridle kites on a regular control bar.
The whole point of fixed bridle kites is that they're simple, and handles work well with them because they have so much freedom of movement. You can slack or tension any particular line almost totally independently of any other. This level of control is why handles are the de-facto choice for the vast majority of fixed-bridle kite fliers. Seriously, consider some handles.
In comparison, the freedom of movement offered by bars is very limited. You need to get everything set-up beforehand so the kite flies well within the limited range offered by the bar. As you can see, it requires a bunch of experimentation and fiddling about. And since most fixed bridles are meant for handles and they don't behave well when flown exclusively off the brakes, most people don't bother.
By putting the brake lines to the middle, the Peter Lynn bar you're looking at basically turns your 4-line Adam into a 3-line like the HQ Hydra. You could do the same thing with your Gaastra bar if you wanted, but it turns the chicken loop into a safety line and relaunch handle. All the power of the kite will be transmitted through the bar (which can get tiring) and as you let the bar out, it actually powers-up the kite up a bit more before stalling and reversing the kite--opposite to the behaviour of depower kites.
I really dislike 2 and 3-line foils because they put all the power through the control bar, which I think gives beginners the bad impression that they need to muscle a kite to get it to do what they want. I believe this is the leading cause of 'bar-against-the-chicken-loop-osis'. In order to avoid this, I use the following progression:
1. 4-line fixed bridle on handles
2. 4-line fixed bridle on handles, with strop and harness
3. 4-line depower foil on control bar
4. 4-line SLEI in the water
The handles are great to start with, as they provide excellent feedback in the hands. Once competently flying the kite, you can give the arms a break and hook-in with the strop between the front lines. This introduces managing the power of the kite with the body, and piloting it with dexterity. The depower foil introduces sheeting to manage the balance between power and responsiveness. Then when it's time for the inflatable, all the sensations and skills are familiar, and the focus can be put on the in-water skills: body dragging, self-rescue, body dragging with board, etc.
I've only got one person up to step 4 thus far, and a handful to step 3. But overall, the results are encouraging and the feedback has been positive thus far. Those at step 3 are mostly female, and mostly friends and family. The one dude at step 4 is an acquaintance who's found it's worked well for him; each step offering new challenges without being too much to manage. He comes from a surfing background with zero windsports experience. He was able to body-drag upwind (well, at least not losing ground) in straight onshore conditions with significant shorebreak on his very first try. I was thoroughly impressed.
You'll note there isn't a fixed-bridle on a bar in there. I was going to try it as step 3, but I ended-up getting the depower foil before anyone reached that point. While I'm still curious how my fixed bridle kites would perform on a bar, trying it out doesn't rank high on the list of 'things I want to do with a kite'.
So, getting back to your personal situation and to answer some of your questions more directly, what to do? Were I in your shoes, I'm not sure how much money I'd want to invest in the struggle of getting a 4-line fixed bridle kite to fly on a bar. Yes, the Adam can be steered off either the fronts or the rears, but the Peter Lynn bar doesn't really offer anything you can't do with your existing kit already, and the Turbo bar is expensive. The price difference is because of the fancy pulley system on the Turbo bar, it mimics the way one files a kite on handles--it's also pricey because it's a low-volume niche product. The number of people who want to fly 4-line fixed-bridle power kites on a bar is few.
Your conversion between the setups for the NPW and the Adam(s) shouldn't be too onerous, it's just a matter of adding or dropping one of the centre lines, is it not? You could just leave the rear flag-out stuff for the Adam(s) attached when you fly the NPW. Any other differences in the setup could probably be accommodated with line extensions on one kite or the other.
That said, I'd definitely get a set of handles. That way you'll be able to see how the Adam *can* perform, and will inform you as to how much brake tension it likes to be flown with.
Finally, if you're feeling really enterprising, you can convert your Adam to depower:
The bridle on your production version may be different than on his prototype. Bridle modifications are not for the faint of heart, and probably not something you'd want to undertake until you're very familiar with foils. They are however, a great way to pass a lot of time without even noticing.
To wit, I have an Aurora 15m and let me tell you; you get what you pay for. It's what I'd call an 'adequate' kite. It really reminded me of my gear from 2004 in terms of how difficult it was to use out of the box. It turned glacially slow and was wanting for power until the apparent wind kicked in, and then it was too much with no way to turn it off. I had two brief sessions on the water with it, and two on the ice with skis. I've now done the Niaboo bridle modification
, but haven't had a chance to try flying it since doing the mod. A buddy who also did the mod says it's *way* better, but requires a bit of fine tuning to get it dialed. Perhaps then it will live-up to the hopes I had for it when I bought it as a "poor man's Flysurfer".
Even then, it's still a foil and it will have all the associated drawbacks. I don't mean to dash any hopes you may have had about reveling in the kiteboarding bliss of having a full quiver of cheap Auroras, but it's just not going to play out that way even if you're prepared to spend countless hours tying little bits of bridle. Inflatables are simply more user-friendly in the water. If you're on snow or land, then it's up for debate.