Yup, said earlier it's nonsense as it's personal device to personal device and using private (not company) info given between people and stored on said personal devices.
I guess exceptions might be if you had access to personal data via your role at the company, like HR having a load of personal addresses/phone numbers etc.. but even then there might be good reason - like say a new manager doesn't have the personal contact info of someone who has gone AWOL they might want HR to pass on details so they can give them a call.
On the flip side if you use a mate in HR to get the personal phone number for the cute new girl on the reception desk or to find other personal info about her then that's clearly abusing company data.
It is entirely reasonable for an organisation to reset your password* and allow someone else to access your device in your absence (although does suggest they have some gaping holes in their business continuity if that is actually required). This however should be audited, and you should reset your password as soon as you are back in the office.
I'd strongly disagree with that, it's not reasonable at all, you've mentioned storing the passwords but even just resetting and then using his credentials without revealing the specific password is also really bad practice, it completely undermines security/audit details.
It's completely unnecessary - designated IT people should have admin access without needing to log in as another individual/use their credentials, ditto to accessing OP or any employee's emails - I can't think of anything that should require logging in as a given regular user that isn't yourself save for bad/sloppy practices. Standard in some places for a manager to be sent all the emails sent to the email address of an employee who has left (might even happen from the date the employee hands in their notice) - they don't, however, reply as that individual or use that individuals credentials in any way, their reply is sent from their own address, they log into their own email account etc.. "Hey [client] Brian has left the company but I see this still needs attention..."
I guess some industries get sloppy perhaps but this sort of thing is pretty important in various places - you want to know who made a code change or who accesses a particular system, for people working with banking systems, for example, you might have regular security checks/audits where someone will call and ask why you logged in on X date and made Y change - if user credentials are being used by others then that's a huge issue for any audit as the whole process is completely undermined. So no one should be given permission to log in and appear as another user - there's just no reasonable grounds to do it.