Alright first cube recipes. There's multiple recipes, that do different things.
You have one that upgrades rares to yellows, that's the one you want to use one weapons, because weapons are annoying to gamble on and weapons are the most useful upgrades in general for a single slot(rings can also be and are also more expensive to gamble, but weapon is higher priority due to the chance of rolling ancient).
You have one that is the set recipe which is very cheap and changes a set pieces to another set piece of the same set(can't be a 2pc set, always gives you a different piece than the one you had, cannot roll ancient). This is great to complete your 6pc sets, by using shit you loot or gamble that you already have or that have shit stats.
Finally you have a reroll legendary recipe, which is expensive to do(50forgotten souls iirc) and that basically gives you the same item you put in it, just rolled differently like it just dropped. This is a very powerful endgame recipe you use to get ancient versions of your gear, and especially your weapon(though for the weapon you'd probably keep both upgrading rares and using this recipe to have more chances).
You want to upgrade rare bows to get Yangs if you want to play UE yes. If you want to play Nat, you want to upgrade 1H xbows instead for Calamity and Natalya. You can use dropped rares or crafted rares(obviously crafted eats more ressources, but it's ok in general). You'll most likely want Calamity for UE too eventually, however it's lower priority so if your goal is UE, then definitely roll bows. Use whatever best bow you roll until you get Yang's. Once you have Yang's, roll for Calamity, then roll more bows for an ancient Yang, though maybe you might want to roll for rings instead if you need focus and restraint still. Meanwhile you keep gambling for armor for your set pieces and stuff and then rings for the right rings and obviously looting random stuff from doing rifts.
For the Natalya helm, if it doesn't give you a bonus, just put it in your stash, don't touch it for now, no need to waste ressources on something you're not gonna use. If you do happen to have some other pieces of Natalya you can probably sacrifice whatever stats you're losing on the helm for the bonus. Iirc 2pc Natalya is the reduced cd on rain when you use a skill, which is very strong for a 2pc, so it's definitely worth losing every stat on your helm. That includes 6% crit and 15%dmg to whatever skill you're using. Obviously it'd mean you'd need to run rain, but it'd be worth it. There's no need to reroll it with the cube.
If however your idea was to use the NPC to Enchant the helm to reroll one stat, if that's what you meant by rerolling, then yeah do use this to reroll a shit stat into a better one. Don't waste too many materials doing it though, in this particular case, do it if you have a 2pc of natalya, but not if you don't. A rare with good stats is good enough until you find the right legendaries.
GR50 is Greater Rift 50, it's the equivalent of like torment 11 or whatever. It's not super high but it's high enough that you want solid gear at that point. It's like when you start "endgame" if you want. Imo the good way to know when you're ready for Hellfire amulets is when you can do Torment 10. If you're not in torment 10, unless you have friends running hellfire you can tag along with, don't bother. It's a horrible use of your playtime, in terms of gearing up. You'd get better results running the storymode, and that's pretty fucking shit. Now if you're bored or want to see the ubers, sure you can do it for fun, just don't focus on hellfire amulets until way later.
