Okay the only reason Nash wells is alive is because he wasn’t on his Earth when the multiverse got destroyed, as for the visions to be honest I don’t know.

J'onn J'onzz restored Team Flash and Team Arrow's pre-Crisis memories. J'onn using his powers to give their memories back causes their post-Crisis memories (which they had of the new post-Crisis timeline until he restored their other memories) to be overwritten. So, that's why Team Flash remembers the other versions of Wells they've met.

When the multiverse was reborn, all versions of Wells were mashed together into the mind of Nash Wells. So, because of this, he now experiences visions of various other iterations of Harrison Wells, like Harry and Sherloque, who are usually trying to tell him something, communicating with him (for instance, leading up to "The Exorcism of Nash Wells", Nash sees Harry and Sherloque, and when the latter appears, Sherloque's eyes glow red. He's trying to warn Nash that, in addition to the other Wellses, Eobard Thawne's mind is also tangled with Nash's.) That's because Eobard had Earth-1 Wells's DNA, since he used future tech to assume Wells appearance and impersonate him to build the particle accelerator to turn Barry into the Flash to harness Barry's speed to return to the future.

However, in "The Exorcism of Nash Wells" Eobard is expelled from Nash's mind; leaving Thawne in a non-corpreal form, now existing as negative tachyons. In the same episode, Team Flash concludes that there's no way to separate the different brainwaves of the various Wellses from Nash's mind. As a result, the visions he experiences of other versions of Wells will continue, but Nash expects he'll get used to it.

So pre-crisis there were multiple Harrison Wells from different earths. Since crisis altered the timeline as the multiverse never existing (sort of) does that mean Harry and HR never existed in this new timeline (because how would they be able to come to earth prime if their is no known multiverse)?

@That Boleyn Girl 02 J'onn used his powers to restore the pre-Crisis memories of the heroes of Earth-Prime (Black Lightning, Team Arrow, Team Flash, the Superfriends, and possibly Mick and Ray of the Legends, while J'onn's fellow Paragons (Kate Kane, Barry Allen, Ryan Choi, Kara Danvers, Sara Lance and, due to his meddling with the fate of Earth-96 Clark Kent with the Book of Destiny; replacing him as a Paragon; Lex Luthor) naturally retained their pre-Crisis memories).

In Earth-Prime's timeline, Harry, HR and Sherloque never travelled to Earth-Prime (apparently), but Nash is now the only Wells in the multiverse with a body, so the minds of every Wells have merged with his, which is why he sees visions of them and can communicate with them. Thanks to J'onn restoring Team Flash's pre-Crisis memories, Team Flash remembers meeting Harry, Sherloque and HR (since Team Flash met them pre-Crisis). I can't be certain about all the versions of Wells that are now in Nash's mind, but I expect that they all either naturally retained their pre-Crisis memories (due to the unusual thing that's now happened with them merging with Nash's mind), or possibly, since they all exist in Nash's mind now, when J'onn restored Nash's memories, he may have also restored the memories of every Harrison Wells, so they can all remember their pre-Crisis lives, and their interaction with Team Flash if applicable.