You say it opened the gate, why kind of lock/latch did you have on the gate? Coons can open just about any latch short of one that physically locks with a combo or key...
Also chain link and the welded 4x2 wire fence you used will only deter some predators... Larger K9s can rip that welded wire apart (break the welds), and smaller predators like weasels/minks can simply crawl through it... Coons can also reach in and if they can grab something inside they will rip it to parts that fit though the fence...
And also you said it had a tin roof, but in the pictures their appears to be no roof at all over the entire run **edit it appears you might have poultry wire over the top?** thus any predator that climbs could simply have climbed up and over, or could rip through the poultry wire...
Sorry about your lose, but if you choose to continue keeping chickens you really need to address the design, and make it much more predator proof or it's bound to happen again...