Spock quite often used a mind meld with an unwilling person. In ST09 He melded with NuKirk without a word of explanation except "it will be easier."
Please, allow me.
- It will be easier.
- Whoa, whoa, what are you doing?
- Our minds, one and together.
In Dagger of the Mind he melds with Van Gelder who is in condition to give permission.
Perhaps the worst offence, outside of TUC, is Requiem for Methuselah. He melds with an unknowing Kirk and erases at least part of his memory. Based on Kirk's reaction to Sybok doing something similar in TFF I wonder how Kirk would react to someone who is supposedly his best friend doing something similar and keeping it a secret.
"I don't want my pain taken away. I need my pain!"
I don;t think that Spock's action with Valaris are really that far out of character for him. Perhaps a little more blunt in execution due to his feelings of betrayal but not that far removed from what he's done in the past.
You can try to explain away his actions in Requiem for Methuselah by saying he was acting to help his griend but in that case why wouldn't he also do it in the case of the deat of his brother or his son? In David's case he wasn't there when it happened but that's the thing about memory, you can erase it at any time down the road and it's the same as if you did it right at the time the event happened. As we saw in TUC, Kirk's pain over David's death lasted a long, long time.