exchange, outlook, .ost file, office 365, exchange active sync

i have 3 questions , i am putting as under:

A) user has left the organization, i want to keep the mail box right now for 1 yr so it doesnt get deleted from 365 when AD account is deleted

should i put on legal hold or retention hold? any difference between them

B)  users accepts meeting request on his iphone but it doesnt show up in his outlook 2016 calendar, i just checked exchange active sync is configured on his phone
and push notification is set to automatic, the meeting he accept was from external source and not within compnay, otherwise other meeting shows in his calendar.

anything i need to run like outlook.exe /cleanfree busy, if so do i need to run this command when outlook is closed or in open state

C) I see some users have very large .ost file ,as we have cache mode enabled, they are not online mode. but the users who have large .ost files have shared mailboxes in their outlook

for ex. when i just checked the size of 1 user mailbox having  4 shared mailboxes  in his outlook amounted to 20GB but .ost file of that user shows 27 gb, so if i redo the outlook profile of the user mailbox which is only 5GB , and then add the shared mailboxes in that users outlook will that help? or the size will again come back to 27gb  as the user mailbox size on office 365 is only 5GB , how should i address shared mailbox so that.ost doesnt get fragmented
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Lisa Hendrickson "CallThatGirl"MS Outlook ExpertCommented:
These questions all need their own post.
Mark GalvinManaging Director / Principal ConsultantCommented:
A) user has left the organization, i want to keep the mail box right now for 1 yr so it doesnt get deleted from 365 when AD account is deleted
If you click 'Delete User' in Office 365 it will allow you to convert the Users Office 365 mailbox to a Shared Mailbox, saving the license cost and allowing you to keep the email.

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