You've explained what angels are -their difference in beings from us- and why they can't be forgiven but it still raises that question: how do we know that the other Angels like Michael are still up there? I do't see how these differences between us and angels answers the question about being capable of sinning. If the highest Angel can sin then what more for the ones lower than him?The difference between the saints and angels is that angels aren't human. Coupled with the fact that they were also born into grace unlike the saints, they have a very different nature.
Contrary to what people tend to think, the angels don't necessarily represent a 'hierarchy'. They are more like a 'semi-hierarchy' with simply different roles.
But Satan was nonetheless the highest which undoubtedly is in some way a factor to his fall. The Bible, particularly with Ezekiel and Isaiah, puts him on a remarkable pedestal and then crumbles it to illustrate this.
Are you saying only Angels are capable of sinning in heaven because they are born in grace while humans aren't? This is not fully agreed on because Catholics do believe that Mary was born and given full grace.