I think the intent behind the phrase 日和見 is "one who waits to see how something turns out and doesn't act until it's clear where everything stands." While it can be a result of opportunism, I think in this case wishy-washy is a good translation choice. One can see how a passive and non-committal attitude would translate into the timid mannerisms we see in the otter routes.
EDIT: While I was a little skeptical myself at first, neither of the other two descriptive words are as straightforward as saying they become "barbaric" or "haughty" either. The word used to describe the wolf's personality change seems to mean "gruff and prone to violence," and the eagle's as "taking an oppressive attitude towards others." Given the actual effects seem to be a little different for all three of them, I'm guessing we can just take these as ZUNisms and judge the spirit's effects by the dialogues.
Also wow when did I become one of those people who lectures others on Japanese. I'm sorry if this is too much.
On a completely different note, something cool I noticed in the demo: This is the first Touhou game where the characters have more than one expression for their defeat portraits. Even after being defeated, the bosses still emote like normal instead of being frozen in one portrait. It's always neat to see new touches like that.