Install SEP client and dont reboot

I have two scenarios for this

1. I am planning to install SEP client to a new server and do not reboot the server, will the sep client be active and do its daily job (i.e get the updates, show online in SEPM, scan and stuff)

2. I had scheduled an upgrade of SEP clients via SEPM, in which failed and disappeared on sepm. By checking on those server we found that sep is no longer installed on those server, if i run an install will this require a reboot or how to know if an OS would require a reboot to install SEP