2)Timeout(ms): Since the file transfer will be scheduled the process will be connect à file transfer à disconnect for each process.
When you do the connection form SAP PI you choose to maintain the connection open or to close in each file transfer, you can establish a determinated timeout, however the called "the process" doesn't seem to clear that PI deposit the file, are you sure they don't have a process to pick the files instead to share a directory that a third-sytem like PI deposit them?
I think this isn't correct. You haven't such an option with the SFTP adapter. Here is the help documentation:
If you want to maintain the connection you have to do some extra work. See SAP note 2069078.
My problem is that I have to guarantee that the connection will be closed after each try. But I can't find such infomation in the online help. I have only this SAP note that implicitly indicates this behaviour.