As per the wikis there are crossovers between both the series. So do I have to watch Once Upon a Time in Wonderland to understand Once Upon a Time or can it be avoided fully?
I don't think its necessary. Unless I'm mistaken there's only one character, "The Knave Of Hearts", who crossed over to Once Upon A Time and they've never mentioned Alice. Although I'd recommend watching Wonderland, I found it really good.
No, the main story arcs of OUaT and OUaTiW are completely independent of each other.
But you might want to watch Frozen (not that I have). I can only assume it lends some character development to these new characters, who seem to have been just dropped in on us. I find the back-story that's given to us in the TV show, lacking.
Side note: you should watch OUaT before you watch OUaTiW, or just know that "magic always comes with a price", that there's no stronger magic then love, and you're good to go.