Now as for your questions:1) I don't use mercs personally. It's just a feature I was never interested in even in classic. That being said, yes they are useful and no I cannot give you any recommendations. The topic has been discussed many times on these forums however, so searching will bring you some ideas.
2) ilvl 99 gives the best chance of rolling tantrum + good affixes. Also bear in mind there is a lot of selection bias going on here. The people who are unlucky or even extremely unlucky in the rng are more likely to complain and post about it, and the people who have average luck will usually not make posts. That's why there are a lot more posts like "I used 2k shrines and no tantrum" (with <ilvl99 bow), rather than "I used 300 and got mine".
3) If you mean the weapon then no, read the second paragraph in the early game gear bow section. If you mean armours then yes - jewels give best performance. Feel free to substitute with lum runes when you get good at the build (for example the early game video recorded is actually using lum runes). Always Jah in jitan's however. This info is all in the guide.
4) Arcane crystals - pick up tiered uniques while progressing through the difficulties and then while doing TA runs on destruction. Signet runs - not necessary just pick up sets and disenchant them as you go. If you must grind out 500 first however, Nihlatak is the best place (use a ring with RIP modifier cube recipe).
.Should i keep rolling until Knife/Arrow Affix? But anyway here is the picture(It is blessed with abondened shrine btw).
