There is no way to talk about end to end connectivity quality assurance.
The understanding must be that a provider can ony control the own
network. How traffic is routed outside is mostly uncontrollable, because
You say you can control your own network, but I suspect much of it is
based on telco supplied lines where you don't directly control the
reliability. This is why companies have contracts. Within certain
parameters (and for a price), they allow you to control the actions of
other companies (and their networks).