Can I (setting aside the obvious secular punishments for a moment) murder someone with a bomb activated via one of these switches?
In the case of murder, the result and intent is prohibited, so no matter how you get there, that's a sin.
In the case of having a fire on Shabbat, there's nothing wrong with wanting a fire or having a fire, the result is not prohibited or a moral wrong; but in that particular time and place you are prohibited to light a fire.
A hundred years ago the standard solution was to simply have someone else (non-jewish) do it, e.g. local gentile boys used to spend saturday mornings doing minor chores in jewish households for some tips. Nowadays it is done with technology. Murder is inherently wrong, so having someone else do your murdering for you is also wrong, but these actions are merely prohibited for you personally so having someone else (or something else!) do it in your stead is considered just fine.
That sounds like a bad incentive. If a shabbos goy decided to convert he would have to stop being paid, which means that you would be paying the shabbos goy to not convert.
Judaism is for the Jewish people, and while some individuals may join (e.g. in marriage) the Jewish people, from the perspective of Judaism it's perfectly okay if all the non-Jewish peoples (i.e. most people in the world) stay non-Jewish forever and their relationship with God is different.
Regarding the loopholes, well, in Rabbinic Judaism the notion is that there simply can't be any loopholes there. Unlike laws written by humans, the word of God is considered axiomatically perfect and flawless - if you find a "mistake", that can't be a mistake, that's fundamentally impossible, God does not make mistakes. There simply can't be an accidental hole or a mistaken omission. The letter of the law is considered literally divinely perfect, every nuance or "loophole" is considered as intentionally placed there by an all-knowing being that knows all the future consequences of that nuance or "loophole". If God wanted the law to be slightly different, He obviously could and would have made it different, but He did not, so it should be interpreted exactly as written.