2. The bug seems indeed be related to the sending sim of a mail disappearing from the world.
Even besides the grammar, that's not right. When my sims receive 17 boxes of crap in the mail it can't possibly all be from mysteriously vanished sims. If so, the "vanishing issue" would be very noticeable.
Please excuse any error in my english, it's not my mother tongue.
The mailbox of my sims is filled twice a sim day with love letters/love presents. The average number of such mail in the box per day is 5. Mail senders include sims you never met - the only precondition seems to be that the attraction system paired them with your sim. If only one of those mails is sent by a sim that vanished afterwards your box will be broken and awesomemod will put your game into an endless loop. Overwatch will get rid of the trouble-making mail and will give you the rest into your inventory. Plain EA code will cancel the "get mail" action and reset your sim next to the mailbox, leaving the box in its broken state.
That is what I observed so far.