To make the 1500 fire first, you'd have to cut down the triangle on the xbow trigger that moves the catch (which can be built back up later per the guide if needed), and not leave any slack on the pull wire for the 1500. You'd have to do this with really tight tolerances though, making it quite the bitch. Figure out how far back the pin pull to fire is, then figure out how far the full pull is. Then shave down the aforementioned triangle just enough so that the xbow fires at the end of that pin pull. The problem is that there won't be any definitive line as to when the xbow is about to fire really. The only difference that you feel is the light catch spring, as opposed to the xbow firing first where you feel the heavy pin spring.
As for the methods of going about all of this, very little of this is original (1500 linked trigger maybe, though there are probably other writeups with that, the trigger grips definitely), and the guide isn't strictly for restoring this. Any of the mods can be pulled out of the guide and applied to a bone stock xbow.
You're fighting yourself here. The reason for replacing it is that it's outdated and virtually useless besides where to reinforce in the most basic sense.
I don't think there's ever going to be a reason to replace Rag's article and system. True, basically every crossbow that's actually been modded, really, is a "Level 4" 'bow. But for the time, it was a legitimate system because there were still a fair number of people using stock darts.
The other argument you pose:
Is really a bad reason to not update it. It's like saying "There are a lot of people who use Windows ME, so there's no reason to make a new Windows." Anyway, I don't think there are really that many, and if there are, I'm sure that they're accustomed to doing mods to make the barrels they want - they can still use all of the information I posted.
Even today, there are a good chunk of nerfers who use stock darts and crayolas or something of the like.
Yes, I've said it before, e-tape is bad, and should only be used when a proper plunger head can't be used/found.
Anyway, I do recommend the plunger tube replacement, though it is offered here only as if necessary. The straight bore combined with the stock plunger head making a significantly better seal without any work, the ability to add a very good amount of padding to the plunger head, as well as being able to place the coupler directly where you want it, without glue, makes it more than worthwhile.