This idea is not new. The theory of Ezra as a ST villain was making the rounds even before Kylo's name was known. It's just as flawed now as it has ever been, so I'll attempt to put it to rest.
1. Watching Rebels should not and will not be necessary to understand TFA
The strongest evidence against Ezra-is-Kylo is plain common sense. A show watched on average by about half a million people will not contain information vital to understanding Episode VII. Abrams has even said that the other saga films should not be absolutely necessary to view prior to TFA. It's clear that the ST is telling its own story, and is designed to be accessible to all viewers. Was further material beyond the OT needed to make those characters work and provide adequate backstory? No. If TFA is well-written, Kylo won't need an entire TV series for setup.
The idea of majorly tying Rebels to TFA as part of some scheme to get more people to watch Rebels is... faulty to say the least. If Disney was banking that hard on the show, it wouldn't be on a tertiary cable network.
2. The development timelines of Rebels and TFA don't match up
Rebels began to be developed in early 2013 and "[b]y December 2013, the production crew had finished developing the series' first season and had begun animating the first script.
Abrams and Kasdan took over writing the TFA script at the end of October 2013. The way Kasdan discusses "hash[ing] out the story" with JJ, with no mention of previous screenwriter Arndt at all, suggests the story was drastically changed from whatever existed previously.
The timelines don't add up. Given JJ's repeated comments about Disney not micromanaging his film, it seems unlikely someone was saying "You can rewrite the script, but your villain needs to have black hair and collect relics so he matches Ezra." All evidence suggests the Rebels characters were set in stone significantly before the TFA characters, so it is extremely unlikely that Rebels was designed as a way to introduce a TFA villain.
3. The higher-ups at Rebels have repeatedly downplayed the idea of significant connections between Rebels and TFA
In August 2014, Simon Kinberg (EP of Rebels, creative consultant on VII) said:
[I]n terms of a responsibility for the Rebels to sort of connect to Episode VII? We haven’t really talked about that. And there is such a huge time gap between them that really what we focus on is the connection and responsibility to Episode IV.
In a later interview, he said:
There are characters, vehicles, imagery that are introduced in the show before we’ve ever seen them in the movies, possibly including VII
That suggests there may be small, easter egg-type Rebels elements in VII. It does not suggest elaborately intertwined stories.
Dave Filoni had this to say about a potential Rebels/TFA connection:
...VII is so far out on the timeline from me that I’m not going to affect it too much, and what they do doesn’t affect us too much.
4. Addressing specific points
Darth Vader has had a reoccurring presence throughout Rebels. He knows about Ezra and his power with the force. What is the point of having him in the show other than letting us know this?
The point of having Vader on the show is to raise the dramatic stakes with a more frightening antagonist than the inquisitors. He serves as an indication of progress entering the second season -- the Ghost crew has gained enough notoriety in the Empire to require Vader's personal attention. Vader's job is to hunt Jedi and enforce the Emperor's will as needed, so his presence makes reasonable plot sense. Vader's not a character that needs special justification to show up in Star Wars media; he's always had an iconic and integral role.
Darth Vader wanted a method of preserving a person that would not permanently harm them. It has never been explained (in the updated story line) why Vader was testing carbonite.
Vader's use of the carbonite chamber is explained in plain language within ESB: "This facility is crude, but it should be adequate to freeze Skywalker for his journey to the Emperor. [...] I do not want the Emperor's prize damaged. We will test it on Captain Solo."
Vader needed an impromptu way to imprison and transport Luke, and the carbonite chamber was the answer. The dialogue has a clear implication that he hadn't considered nor attempted freezing people before.
Darth Vader has been known to secretly recruit sith in the past (Starkiller from Force Unleashed).
The Force Unleashed is non-canon. As far as I know, Vader had no canonical secret apprentices.
It is also very important to note that the writing teams on the show and film are linked.
No. TFA was written by Kasdan and Abrams. Neither has a part in writing Rebels. They're only linked by the story group, in the same way all Star Wars material is "linked." This does not mean they are intricately tied to one another, merely that someone is overseeing overall continuity.