I'd suggest using the moisture sealing electrical tape/rubber slicing tape, same stuff Petzl uses and sells with the Nomic.
Couple of brands available at Home Depot or Lowe's. 3M and Scotch brand. I like the thicker Scotch brand stuff on my Nomics and the thinner stuff on some of the other axes. All of the tape is cheap, sticky when wet and pretty durable.
Trigger? Most manufactures have stopped offering triggers on their tools. No matter how they are adjusted in use they tend to stress/rip finger tendons, made worse yet with cold hands and even more chance of injury. The real advantage isn't the directional or limited additional support of a trigger anyway but of a higher grip to match on and do fewer placements. You'll want a bigger platform than a trigger to do that, something like the Slider that can support your entire hand. What ever you decide to bolt on it will work better to add the tape first.
Lots of ways to attach umbilicals this is what I thought the easiest answer. I drilled a hole just big enough to hitch a spectra QD through.


The biggest improvement you can make in your own climbing on steep technical ground is ditch the leashes, add a second, higher, grip to match on and use umbilicals.
Lots of tools out there. Making a tool into a leashless tool, is a start but sad fact is you really don't have a tool designed to climb with leashless.
Not something anyone wants to hear but the best tools designed from the start to climb leashless really do make ice and mixed much, much easier. Hard to make a pommel tool like the Nomic or Fusion at home