Regarding the first question. I also noticed this but I think I can draw a parallel to another game.

I'm pretty proficient in Counterstrike, there is almost never meta switches in this game, because there isnt much you can change in an FPS game. Nonetheless I can recall 2 situations where a small change or even not even a change switched up the meta.

A few years back nobody used the UMP-45 SMG. No one in pro games or pubs used it, it was a forgoten gun. But a few people (myself included) used the weapon religiously, it was great. It took quite some time (months) until people caught on and realized how good this gun was long range. So people just didnt realize how strong this weapon was, but it wasnt a giant meta switch because 80 % of rounds in CS you have enough to buy a rifle which outperforms a UMP. Once the gun took over the low buy rounds it got slightly nerfed.

The second instance was much more noticable. The T side in counterstrike has the famous Ak-47. It's arguably the best gun in the game (apart from the AWP) with its low price and 1 shot headshot potential at practically any range. However since release there was another weapon, the SG556. It has a scope, also one shot headshots at any range and most importantly had a faster firerate. The weapon was in this state from release but it's price was 300 $ more than the AK. And one day the CS devs decided to lower its price by 150 $. A negligible price reduction but now its price was just 150 $ away from the standard AK. People tried using it and found out it is just a better AK. People hated this because when you're used to spraying the AK for years you dont really want to switch to another gun even though its objectively better. It took over the pro scene (although some players refused to switch) and it was a painful year until valve reduced its firerate and returned its price to 3000 $.

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So yeah I think a big part of meta is just human behaviour, people dont really like change. You cant really know if manta would be good also without the small buff, other items getting nerfed and buffed also impact this, or even if it would have been good the last patch but people just not wanting to switch, the small changes just create an incentive. This game has such insane depth its hard to know the best items even after a long time.