Who needs permafrost xD. That's not mandatory even though it does help . It's not always needed though as seen with my Frozen build.
Is reactor worth it in PvE? Debatable tbh but if you don't like it , then you can replace. Keep in mind with the insane DMG of my rogue orb Discordance Chakram build, the reactor does at least 62.5% of that 8000% MH so around 5000%+ MH per 0.5 seconds, hits up to 5 enemies and a decent hit frequency as a well as an easy way to freeze many for survival and cause frozen chain loop.
Yes reactor set can be gotten in amethyst on wizard items (or Jasper crystal other class items to wizard, then Jasper back to your original class with the crystal).
However you won't benefit from Nadroji and Eternalized on hirling or any set affix on hirling, except very few.
Reason being is that most set affixes are only applicable to one character only and most affixes in general.
The exceptions are: Epiphany since it affects cap of Gold find and Luck and Item drops per character but due to averaging formula, it needs 2 epiphany (5) on each character to increase the cap fully by 25%; gold find , luck, and item drops because they are affected by averaging so you need them on both characters in hirling setups but not in solo.
The other exceptions are debuffs. If an affix debuffs, the enemy will be affected in a negative way and vulnerable to any type of DMG for eg or slow down.
Stack Debuff can stack even with hirling so if both characters stack Debuff with shock 4 times, you end up having enemies with 8+ stack Debuff and vulnerable to 100% more DMG instead of 50% for eg.
I haven't tried this but Ice Debuff of slow down could theoretically stack with both characters to stop the enemy move speed (unless they are faster than normal) and maybe players ,unless they have move speed skill/affix . It probably could also stack the DMG Reduction of ice from 25% to 50% .
Although it's more than likely not possible since it doesn't sound like a stack Debuff like shock is. Still if one character slows down enemies, the enemy will still be affected to deal less DMG to both characters or players rather and the slowness does help. Not really necessary though but it's something that could be used in 2v2s or campaigns.
Ice Frozen is a Debuff in itself though but in Frozen explosions, the frozen explodes for the applied so only the character that has Frozen and Frostbiting or so. Although it can also be possible that the other characters, provided they have Frostbiting and enough DMG and frozen set could affect the hirlings frozen in some form whether for good or bad. Shock+ Frozen does work well though.
Another Debuff is arcane aka ascendant + wrath /iceburn or any arcane attacks. Arcanist is potentially one too as the reason why I used ascendant before 3.0 on the hirling was to increase the main character total DMG% when they have Arcanist equipped.
Arcanist effect has changed to increase ED% by 20% on weakened enemies and arcane Debuff but it's possible that it still probably works if hirling has arcane Debuff and weaken maybe. I haven't truly tested but so far, barely any difference worth mentioning . The Increase ED% would be exclusive to the character who wears Arcanist only though like how Total DMG% would be in old Arcanist.
Ascendant is definitely a Debuff though and it can probably affect both characters like a high enough weaken % will do that if hirling places debuffs of weaken/ascendant or both , the main character attacks benefit. It wouldn't work if the character had ignore resist though. Although it's also probable that weaken only affects the user who wears it on the enemies but it sounds like it should affect both characters in a form.
So much potential in the game I could try out or already tried and still even with lots of crystals, easy to run out eventually.