Hmm, well you are rooted, if you have the full version of Titanium Backup you could simply "Freeze" the stock Messaging app so it was disabled entirely and see if THEN the widget properly throws you to Hangouts...however, I'd have expected it to prompt you for what app should handle SMS once you cleared the defaults from the stock app. Especially since installing another SMS app did make the system ask you. I use Handcent SMS still and the old Talk app, but if I clear defaults or even update Handcent, the next time I click a "SMS to Contact" shortcut on my homescreen it asks what app to use the first time. Certainly try reinstalling Hangouts and also making sure it has handling of sms checked off too.