Simply put: Skills.
First level gives x4 skills vs other levels. Also it gets you access to the trapper skills and umd so you can maintain them when taking other levels. The only reason not to take rogue or arti at first level in a multiclass is if you are in an iconic and can’t.
When do I take my second trapper level? I find the sweet spot on a 2 splash to be no earlier than level 8. It can vary based on your int and tomes, and later is always better for skills.
Not at 2. I know you want that evasion/rune arm, but don’t do it. You don’t need it yet and you sacrifice skills by taking it early. If you are taking a trapper level in a light splash and aren’t spending all your skill points on trapper class skills you might be taking it too early. Remember skills spent when taking a trapper level are at cost, not double cost like other classes.
Personal priority on trapper skills outside of any other requirements of your build. On a full skill build: search>disable>open locks>UMD. For a partial skill build Search>open locks>UMD. Depends on what you demand, and what tomes you have. UMD will usually hit the target of heals/res with a smaller investment. Locks target for me is the shortcut in House of Death undone, and search target is the right door in HH.