Look, the reason why there's a fuss about the instamine.....They cannot afford to have centralized screw-ups like what happened with Dash's instamine
I doubt very much that that's the reason.
I think the reason is plain and simple - that many people feel it's unjustified that a coin with Dash's stumble into existence be favoured by markets. So their problem is with the market, not with the product, its developer or followers.
Coins who's valuation is on the deck don't get this kind of attention.
It might be justified if Dash was the only cryptocurrency in existence and one in which everyone therefore had an interest, but it isn't. It's one of hundreds which are all independent of each other. The early launch period may have been unfair to a tiny handful of miners who missed their chance to mine during a few hours (a valueless coin at the time), but it wasn't to anyone else - especially not those who bought it in markets and held an asset that had huge amounts of work done on it during subsequent months.
This is not a game with rules. As you are pains to point out, it's a decentralised free market in which developers are free to tweak whatever parameters they like subject to 2 conditions:
[1] - their revisions are accepted by a majority of miners
[2] - their actions expose them to the risk of market revaluation
Those are the only "referees" and Dash is just as subject to their appraisals as every other project in this space. Emission curves are not a question of ethics (at least not for the industry) they are a question of convention, otherwise cryptocurrencies could not properly be called decentralised.
The hypocrisy of the near-hysterical 'instamine' rants is that Dash did not have the current valuation when the instamine (low difficulty + slow retarget or whatever you want to call it) occurred. The current valuation has been achieved through the hard work and innovation of many, but significantly the same person who is under perpetual criticism for the bad launch.
So there is no "centralised tinkering" that was not subject to the conditions above.
Furthermore and contrary to bitcoin's convention, Dash has a stated policy of tinkering with "everything" at an early stage to attempt to "get it right". Whether you think that policy sensible, ethical, commercial, sustainable or not, the dev is perfectly at liberty to pursue it and discover the consequences be they favourable or adverse.