1). I’m not going to acknowledge this because read my posts I discussed Alain’s late arrival to the anime and the spacing between the specials and the regular series. Never said there wasn’t a possibility they wouldn’t meet so I don’t know where that’s coming from.
2) Marin mentioning that Ash assisted Garchomp isn’t an allusion to them meeting in fact it’s basically an Easter egg.
3) Again how is this proving Alain and Ash where going to meet?
4) Finally the opening is something that occurred externally out of the actual show. Openings are suppose to contain spoilers but the special themselves did very little to connect the dots and make a secure connection with the current anime.
Literally everything you mentioned still could of happened and the specials could of still been this independent thing that didn’t intertwine with the main series. They didn’t have to merge until the writers forciably did it in the last hour. That’s what I’m arguing; not the possibly of the writers not knowing they’d include Alain or not and idk where you got that from. The most of I’ve said was that his arrival that late into the series probably wasn’t planned throughly.
If there is one thing I've learned regarding theories, it's that the simplest explanation is usually the correct one. Individually, they don't seem to prove anything but take them all together and you get a consistent pattern. Alain is part of the Pokémon Anime Universe thanks to Marin mentioning Ash in the first Mega Evolution special. And if that wasn't enough, the main series showcased Alain as Sycamore's assistant in a flashback. And if it is part of the same universe, chances are there'll be a crossover in some capacity whether it is Misty's Gyarados or Dawn's Quilava or Vincent/Jackson in the Johto League. That's the premise, the two journeys are in the same universe and there's Team Flare in the background. Inevitably, Ash will fight Team Flare just like he did with Team Plasma, Galactic, Magma, Aqua and of course, Rocket. And Lsyandre happens to be Alain's boss, also introduced in the first Mega Evolution special. You honestly think it didn't cross into the production team's mind that they would have Alain cross over with Ash the moment his character was conceived?
Then the opening outright teased that Alain was going to jump into the main series. Openings like these spoil certain characters and events all the time. It's how we knew in advance that Goomy was going to evolve quickly or that Ash got a Torterra of all Pokémon in DP or that Sawyer was going to be the underdog rival. Alain's segment happens to replace the Clemont rivalry with Ash, meaning that he's the next real rival for Ash to face. All in five months before the official announcement of XY&Z.
Additionally, there was
Mairin's Mega Evolution Journal which aired as a post credit segment after every XY episode starting in XY042 in September 4, 2014. It's as if the main series wanted to remind everyone that the specials took place in the universe over and over again. This was unheard in any previous Pokémon special episode.
The Legend of Thunder never had anything like that.
So when you add of these instances together, look at the timeline when these instances started to occur, you start to see one overarching narrative: The Mega Evolution Specials was going to cross over to the main anime. It's not a matter of if, it's matter of when.
There was no last minute changes, no executive meddling. Yajima wasn't lying when he said that Alain was always going to enter into the main series. The instances date back in 2013 and 2014 and 2015, meaning that Alain was never going to be some separate spin-off project.