1. Has never happened in the history of the game with the ONLY exception being NIN, and that was a special case. They're having Relics added (as are SGE) right now. I think they also may back-date the Ultimates by making weapons for the new Jobs for them, like how they made Thordain Extreme now drop RPR and SGE weapons since they added the models for them for the Dragonsong Ultimate and, as the models were now in the game with the special visual effect, they can put the matte version into the Extreme Trial's loot table without issue. You may think this is a thing, but it's not, and never has been. Period. (Aside: I hate when people say something and then say "Period", as if that means they get the last word or everyone must bow to it. Tends to generate an antagonistic response...)
2. NPCs in the story often wear sets. Thancred and Urianger outright wear the GNB and AST sets. Alphinaud wears the SGE set. Estinian is still in level 50 DRG gear. This is what is typical for new Jobs, but it expands as they're in more expansions. Moreover, you have access to all past Maiming sets, so it's not like you have no other gear options, and you also have all the level 1 glamour gear with no stats that can be used on any Job, or all the PvP sets for Maiming. And I imagine Reapers wearing medium armor. That is, in fact, the first thing that comes to mind. Like how DRK's wear plate, I'd expect RPRs to wear mail or scale armor, with some options for leather. Which is what they have. And no, they should not "be allowed to wear anything that a tank can not wear". No other Job gets that privilege, and seeing a RPR in Caster or Healer gear would look even more ridiculous than "a Samurai with a scythe" looks. (Never mind most SAM gear IS cloth/leather, the thing you're asking for... <_<)
3. Third verse same as the first. This is true in all expansions for all added Jobs. They aren't going to make a ton of gear that is useless. The reason all those level 60 vendors have WHM gear and not RPR gear is because people are still level WHMs in that content and need those as actual gear upgrades while RPRs start at level 70 and so have no need of gear below it. That said, a lot of low level gear HAS been made where RPRs can use it, just not weapons because that would require them to spend hours of Dev time just making various flavors of weapon that 99.99% of the playerbase won't ever use or even see. That said, some old content has weapons backdated to them, such as Palace of the Dead and Heaven on High both having two stages of weapon for all Jobs (except BLU) in the game, including newly added ones since your character automatically equips those weapons when you zone in. The copied armor in PvP is usually different. It may be the same model but likely has different dye setups. SGE has the same thing.
Archfiend set is very recent and is with the PvP Season system. That's just the way it works, but it's expected to be obtainable again in the future. Indeed, the ONLY gearsets that AREN'T obtainable are those special PvP ones. Of which there has been exactly 1 to date. So you say "best armor pieces for Reaper", are you talking about A SINGLE SET of armor? Because what other armor sets have been made unobtainable? The False Monarchy will be after this Season, but that would bring the total number of unobtainable sets...to 2. This sounds like a complaint you're using just to pad your complaint list. Not only that, the Archfiend set is heavy PLATE armor! The very thing you just got through saying doesn't fit RPR's aesthetic. You need to stop arguing against your own argument as examples for why your Job is slighted...
Take a look for example at the Void Ark Mail of Maiming gearset. It's not heavy armor, it's more light like mail, and would look fine on a RPR. (It's also something you can farm fairly effectively during Moogle Tome events when its one of the featured Duties.)
I'm also REALLY curious what you think historic scythe users wore? Historically, scythes were not weapons of war, they were farmer tools. A modification of it, the Kama (a short, one-handed scythe) was used by some assassin type groups and irl ninjas, sometimes with a chain attachment, but it was a rare weapon and difficult combat style to teach and learn. There weren't a lot of historic scythe users, and nothing like anime scythe users since the weapon is so unwieldy and requires a lot of understanding and training with rotational momentum to use effectively. Most people with access to a scythe could just as easily make a staff or a spear, and both were easier to use as weapons. So people that DID use scythes in combat were probably garbed like either LNCers (not even DRGs), and obviously RPRs can wear all the armor that LNCs can. Most war scythes were more like halberds, having the blade put on the long axis instead of 90 degrees. They were used as such by peasant armies, meaning again, you're talking about dressing like a farmer or MAYBE a LNC/Wood Wailer in padded or light leather armor. And again, plenty of those gearsets exist from ARR, and you can dye them black if you want a darker aesthetic.
I'm not sure what history you're appealing to, here...