Reason why FF15 development was a mess was because first it was being developed for ps3, changed engine, then started developing for ps4, Luminous engine not up to snuff lot of technical problems, they dropped using this engine on Kingdom Hearts 3, they redone the whole game as they didn't like the Story etc.
Irrelevant. As I said, if we pretend the development started with the change in title and the porting to the Luminous engine, the game, by time of release will have been in development for 4 years, and that's with a lot of the systems, designs and resources having been worked on for several years prior.
In the case of the remake, everything is being made from scratch, which still punts it further back the line in terms of development than the point FF15 was at once the porting began back in 2012.
They haven't redone the entire game - A lot of the dev videos explicitly states that a lot of the work they're doing is porting the systems over from one engine to the other. Yes, they've re-written the story etc. but the leg-work of general designs, animations etc. seems to have carried over - and as anyone who works with game-development can vouch for, that's a much bigger part of development than say scripting story events or recording new dialogue.
I'm sure if they remake FF7 they would remake it also in Unreal Engine 4 just like they're doing with Kingdom Hearts 3, Dragon Quest XI. Would speed up development alot
Not really when you consider that the Unreal 4 Engine hasn't been used for a single open world, large scale RPG to date, and is a relatively new engine. Looking at Unreal 3 you won't find a single open world RPG for that either.
The only relatively noteworthy RPGs on I found for that engine (U3) was the Mass Effect series, and The Last Remnant.
That being said, Mass Effect 3 began development before Mass Effect 2 was even released giving it a development window of 3+ years despite being developed on the same engine as the 2 previous games by the same company.
The Last Remnant which is short, linear and not even close to being in the same camp as larger open world RPGs, still had a development cycle of around 2 years.
So, regardless of whether it's in-house or over at CyberConnect2, we're looking at a team working with a new engine, producing a type of game the engine isn't necessarily well-adapted to making, so it while it may be faster work than on the Luminous you have to take that into account, pluss the fact that they'll be making all the resources and systems for the game from scratch this time around, unlike FF15.
As I said - unless we're looking at really drastic changes and content cuts, this game isn't going to come out a couple of years, and letting the remake be stuck in production for 3+ years granted the gaming climate in Japan and the economical situation of SE is very unlikely.
It might be uncomfortable to think about, but it's getting more and more probably that what we're looking at here is a stream-lined, action-oriented release, where the predominant portion of the game-play and presentation will be more similar to that of the 13 trilogy than that of FF15, or earlier FF games.
While they haven't said what engine they are using, they have already confirmed for certain they will not be using Luminous Engine. So no need to assume 
That's not what I meant by that statement - I meant that I am not willing to assume that it's necessarily more difficult for SE to work with the luminous engine in producing open-world RPGs, than for instance the Unreal 4 engine.
Since most games on Unreal 3/4 tend to be action/adventure/shooters, not RPGs, it might very well be the case that making an open world RPG on Unreal is complicated and time-consuming, maybe even more so than working on the Luminous engine which was designed expressly for that purpose.
