1: Then that's an issue of them taking feedback but getting the implementation wrong. Barb EX was quite well received and was also 'more healing' but in a different style (faster, not harder), so the question is if they try another direction of approaching 'more healing' or just give up on the whole thing because their first attempt failed. I'd like to hope it's the latter considering how many times they tried to get Diadem to be accepted, but I don't have much optimism left to spend
2: You absolutely 'can', SE 'doesn't', that's the difference. Again, WOW did a fairly largescale rework to Ret Paladin (which had some extra knock-on effects to Prot and Holy, which later got fully polished out in the patch after), and to Mage, in 10.1 and 10.1.5. Just because SE doesn't want to, or hasn't in the past, doesn't mean they can't, it just means they don't want to. And I don't expect that to change, nice as it would be. I'm also not sure what is meant by 'completely upend the battle system', I'd reserve that term for things like adding Interruptions as a mechanic, or Shield Check mechanics (with an actual UI element), not 'SCH got two of it's DOTs returned'
3: We get datamining anyway. WOW PTR is not datamining, the mythic phase of the last boss is always hidden because that's the main 'viewership draw' of the race. Even if we could see every mechanic's debuff via datamining before the servers went up in FFXIV, it would tell us almost nothing. The descriptions on the A/B debuffs in P12S literally say 'Marked to be the target of a specific attack.' That doesn't tell us anything about what the 'specific attack' is, how much damage it does, how long this A/B debuff lasts, all we can derive from it would be that A does something, B does something else, and that much is obvious from literally one pull getting there
The playerbase being daft about potency changes is not a reason to withhold all changes until the expansion. Leaving it out of Preliminary Notes, sure whatever I don't care, but holding off on changing AST which has been said to be in 'such a state that it needs a big rework, big enough that it can only be done with an expansion', sounds like it should be given at least SOMEthing in the meantime??? 'Yeh, it needs so many changes, we can't do them in one patch, so we are going to leave it exactly as it is for a year longer than we originally planned.' Not even a small QOL like giving Lightspeed a second charge to at least tide the poor bastards who main the job over till 7.0? Really?
I'd do even smaller: I'd just have a client that has the server built-in, and it connects to 'localhost: portnumber' instead of an actual server. Then, just have it preloaded with one character's data on the server: John Midlander, max level everything. And loading into the game puts you in one map, with all exits disabled: Middle La Noscea. Your home point is Summerford Farms where the dummies are, you have zero gil so you cannot TP, and the zone transition lines are blocked off. Or a special, empty room with dummies set up in formations of like, one, three, five and ten, to test various ST/AOE scenarios. All you do on the client is test whatever thing SE's put on there for people to test. People can infer most (like 90%+) of the gameplay from combat with dummies.
They have that benchmark program, after all, so it's not like 'standalone download that doesn't access SE servers during runtime' is alien to them. Make a PTR download that runs a 'server' on the client's PC for just that client to access, throw whatever gameplay changes to test onto there, asking 'hey we were wondering if people would be interested in WHM having a Water spell as part of their rotation, give us feedback on site where you downloaded this from', and see if people say 'its good' or 'its bad' or 'I like the idea but the MP cost/CD/gauge cost/etc is too short/long'