I can't help with the deck problem; I don't have the kit. The bottom line on such things is: whatever way is easiest is the best.
I'm not convinced that running wires up the inside of the masts will accomplish much. Again - I don't have the kit, so I can't judge how flexible the parts are. But I'm inclined to think that all except the thinnest of them are sturdy enough to do what's expected of them - and the thinnest ones are too thin for the wire treatment. If a particular mast or yard is so flexible that it seems incapable to taking the strain of the rigging, you might consider replacing it with either wire or wood.
I suppose either copper wire or steel wire could be used for the standing rigging, but I don't see any advantage to it. And there's one big disadvantage. Wire kinks easily. If you happen to bump into a piece of wire rigging after you've installed it, it's likely to acquire a kink - which will be difficult to remove.
Working with thread takes some practice; it requires a few skills that other phases of modeling rarely if ever use. But those skills are basic to sailing ship modeling.
It seems to me that wire is preferable to thread under two circumstances. One - where the line needs to be shaped into a realistic "sag" (e.g., footropes). Two - where the line needs to be thinner than any available thread.
Those copper wire sizes you mentioned are pretty heavy - probably too heavy for all but the biggest lines of the standing rigging. You could find some finer copper wire (individual strands pulled from multi-strand electrical wire, for example), but I don't see how that would be preferable to thread for the standing rigging.
The term "fishing line" embraces several materials. I don't recommend nylon monofilament; it's difficult to tie in knots, and doesn't really look like rope (or wire, for that matter). Some of the other synthetic fishline might work but, again, I don't think it would be as good as several other materials on the market.
I have no idea what sort of stores in Taiwan sell thread, but you might want to nose around whatever places people buy supplies for sewing. Normal sewing thread isn't bad for ship models - unless it has a high percentage of cotton in it. (Cotton is weak, and hygroscopic - that is, it expands and contracts with changes in humidity.) While you're at the sewing store, pick up a piece of beeswax. Get in the habit of running each piece of thread over the wax before you put it on the model.
Mail order firms like Bluejacket and Model Expo (whose website is back up and running, by the way) sell a wide variety of thread that's intended for ship model rigging. I don't know what quality of service to Taiwan they offer (or how much it costs), but if you can deal with them that's the route I strongly recommend. You've got quite a bit of time ahead of you before you'll be ready to start the rigging - plenty of time to place an order over the web for rigging line and whatever other aftermarket supplies you need, and get it delivered.
Hope this helps a little. Good luck.