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For the RG/8U I'd suggest Ancor's expensive cable, which has a 96% shield and PVC dielectric material, which will absorb less water and distort less than the more common foams will. It is also fully tinned--which will make a difference since no matter how you seal the cable, moisture finds a way in.
In theory you support vertical coax by tying it off to a support every so many inches to spread the stress. Unless you have some kind of liner to secure your cable bundle to, that's probably not possible for you. Using a steel wire, or copperweld (copper over steel) and running that with your cable bundle, would allow you to tie them all together (maybe every foot or two) and use the steel cable to take the weight of the bundle.
To split out the steaming light, deck light, etc. you would need to measure the length to that point on the mast, then split open the outer jacket and extract those two wires, or split it open and splice in a "tee" if you can't pull them back down. You'd also need to splice in a "tee" to the ground wire in the bundle at that point. I'd make sure the splice was well sealed with butyl tape or silicone tape, and if there are any internal halyards, some anti-chafe over that.
The quality of your work (grommets, seals, splices) will pay off, so by all means go for the best.