Sorry for being a bit late on this…
Got two glowy silicone implants myself, so I’d like to share some information. Pictures are found a lot around this forum, like this:
and this (among other stuff):
So yeah, especially the power button is designed to be (very) visible even in normal light. But that’s exactly what I wanted, so no problem for me - but maybe one for you 
The star, on the other hand, is nearly invisible. There is a little bump on my finger, but unless you know it, you most likely won’t notice it. And it still glows nicely when it’s dark. Both are Haworth implants, obviously, and I think it might be interesting for you to check the “mini” ones - there is a mini power button, which is about 2 mm thick, and the mini stars with a thickness of 1.59 mm. Depending on where you put them (skin has different thickness all over the body), they might very well be invisible, especially on arms (upper side - the underside, where I have mine, has thinner skin) or legs.
But this brings a little problem - the thicker the skin, the less light reaches the implant, so you will most likely lose some glow. In general, please don’t overestimate the glow - it is very bright in a pitch black room or when activated with a UV flashlight, but it’s not like you can see it in a club or a dimly lit room. Just saying that so you’re not disappointed, I love it the way it is 
Depending on the design you have in mind, it might work to just use several of the mini stars, though you will have to take a lot of care that they don’t move during the healing time. Splints, tape, whatever.
Nope, sadly not. Though there are some experiments with glowpowder in this forum (including by myself, trying to do glow in the dark tattoos - doesn’t work as good as I hoped yet, but I’m working on it ). Another person who tried out a lot in that field is @mrln, maybe he has some good ideas I forgot
They do because it was the whole point of silicone implants, until Steve decided to make them glow as well 
There is no way to avoid that, because they can’t be placed deeper or such - they are always fully underneath the skin, and it’s not possible nor healthy to go deeper. The only way to avoid a visible bulge is to place a relatively thin implant under relatively thick skin.
If you have any questions left, feel free to ask - I’m not that active here currently, but I’ll try my best to help.
Oh, on that point - yes, this is theoretically possible, but it might take some (like, a lot of) time. Do you already have an artist who would do the install for you? If so, it might be a good idea to let them contact Steve, because there might already be some connections that could speed it up a bit. But better be patient