IMHO those are not comparable scenarios.

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.

>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

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.

That's perfectly fine, he should not convert if minor tips are an obstacle. Conversion is possible if someone really wants to, but is not really encouraged (and perhaps even intentionally discouraged, different branches of Judaism have different attitudes towards this) - in stark contrast to Christianity and Islam, proselityzing and bringing every soul worldwide to God is not considered an important goal.

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.