If we look at both cards:
Mewtoo has immediate and tremendous value. Mew not, but mew has inevitability. Leaving mew alive for one turn is doable, but probably already game over. However, mew leaves the opponent at least searching one turn for an answer.
Mindslaver, even this weaker variant plays out as follows: either the opponent can play its answer card, if in possession of and mana wise available or they are used against the opponent. Similarly, very bad attacks are made. Hence, having the slaver ability as an etb will work out as follows: oppo looks at the board, hand, etc, and has an answer or very very likely concedes.
You seem hellbent on slavering, but immediately slavering leads to either nothing to slaver or conceding. You also wish that the opponent keeps on playing to find an out. So a solution would be to have the slaver ability later and “answerable”, e.g.,
mewtwo 4
Kicker x put x +1/+1 counters on ~
Flying (or shadow?)
At the beginning of your upkeep add a +1/+1 counter on ~.
At the end of combat, if ~ attacked, you may remove 4 +1/+1 counters from ~. If you do you gain control of target opponent during that player’s next turn. After that turn, that player takes an extra turn.
sacrifice 2 permanents: remove a +1/+1 counter from ~. Any player may play this ability.
2/2
There, a tad wordy but it is strong, has the slaver ability, leaves the opponent playing for an out, wins the game if that out is not found.
Thank you for your input, your concerns are reasonable and you are right. However, a 4 mana 2/2 would just feel too weird for Mewtwo and I don't love the wordiness of your design. So I tried to come up with a simpler solution for the same problem.
Mewthree
Legendary Pokémon - Psycho
Flying, Ward
At the beginning of your upkeep, if the last turn wasn't an extra turn, tap Mewtwo. If you do, you gain control of target opponent during that player’s next turn. After that turn, that player takes an extra turn.
10/7
This way, the mindslaver would happen before they get destroyed by the 10-power flier. And during that turn, they couldn't have their creatures suicided into Mewtwo.