Obviously susceptible to people misplaying it and getting too many +1/+1 counters.

Is there a specific meaning for 7 instances of evolve?

Wouldn't this always become 7/8? (I mean, won't each evolve see the other creature enter, pass their test, and then all add? If not, why not?)

Oooh, this card reveals a bug, too - the "loading..." gif isn't replaced by the real image; it just get overlapped, and shows through the transparent bits.

Nope. Evolve has an intervening "if" clause which means each resolving evolve will only give the counter if the criterion is still met.

The same applies if you have a normal 2/1 evolve creature and play a spell that makes a bunch of 2/2 tokens. Evolve triggers once for each, but each trigger will only give a counter if the condition is still met when it resolves.

In light of Reef Worm, this can easily shave at least 1 mana, possibly even 2. But it won't, because the Magikarp Salesman Cartel likes their wares to be overpriced.

Comparing normal Magic cards to Commander rares is tricky business. I'd assume Reef Worm is the equivalent of a Mythic.