Yes, you need to check for any obvious mechanical problems causing the breaks and I always used an 8thou string for that top G, but the best tip I can give from over 30 years experience of playing a 12 string, is tune the guitar a tone (two frets) down. It saves the strings and takes a tremendous load off the neck.
I think the tension is a 'square' factor. i.e. to raise the pitch by an octave you halve the string length which suggests you would double the tension to achieve the same effect. But you don't it's 2 squared i.e. 4 times the pressure (ouch.)
Sadly, following my stroke, my 12 string days are a thing of the past. My old 'Folk 12' is now in Leadfingers' capable hands.