Looks like you got most of the questions answered here. I too do not know if you can use the two Victron negatives as common? Some units can and some can't. You woun't break anything regardless. Looks like those y cables are too short to allow stain reliefs?
MidNite has used CBI and Carling breakers now since 2000 (at OutBack Power). We ship almost 1 million breakers each year. They just do not fail! Chinese combiners have nice features and all, but do they have a UL listing? Some of the components as pointed out in this thread are questionable too. Listed components are actually important if you have a fire on your house. Insurance companies look for things like lack of listed equipment, inspections and such. They do not like to pay off on fires caused or that are involved with solar. They are terrible! Make sure to cover all bases. It took me years to figure out the combiner requirements and NEC acceptable products /installations. I recently took apart my original solar array from my Trace Engineering days. The combiner consisted of nothing but wire nuts!
