What does Microsoft say about this? Or are they mum as usual?
From what you say in your post, it appears they are trying to fix WLM...or am I reading it incorrectly?
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We have received reports that the Windows Live Mail update- KB KB3093594, is causing stability issues for many of our customers running Windows Live Mail 2012.
Thank you for reporting this issue.
We have identified the root cause and are currently working on a fix that addresses the issue for impacted users on applicable platforms.
Please visit Update for Windows Live Essentials Mail 2012 or this thread for the latest updates.
***Update Dec 20^th^
Sorry for the delay in posting this update.
On Dec 17^th^we rereleased the Windows Live Mail update KB3093594. For users who had previously installed this update and were experiencing a crash when launching the Windows Live Mail app, please download and re-install KB3093594 and this issue should be resolved.
https://support.microsoft.com/kb/3093594
Some users have independently reported a separate issue with sync of mail with attachments that we continue to investigate.
Update 1 February 2016
(posted as Issues 3 under Known issues ... for KB3093594 rev. 6.0)
Some customers have reported other issues with the release on December 17, 2015. This update is no longer being offered while Microsoft investigates these issues further. Server interactions will remain supported during this investigation.
NB: links to http://www.microsoft.com/en-us/download/details.aspx?id=49486 will not work 
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What does Microsoft say about this? Or are they mum as usual?
From what you say in your post, it appears they are trying to fix WLM...or am I reading it incorrectly?
Latest Insider Build 14251.rs1_release.160124-1059 pushed today. OK same screw up.
WLM refused to open and indicated same dll missing problems, directed to WLM.
VS C++ 2010 - https://www.microsoft.com/en-us/download/details.aspx?id=5555
VS C++ 2012 Update 5 - https://www.microsoft.com/en-us/download/details.aspx?id=48708
2015 Update 1 - https://www.microsoft.com/en-us/download/details.aspx?id=49984
Anyway I copied these 4 files.
msvcp100.dll
msvcp110.dll
msvcr100.dll
msvcr110.dll
You might have to be an administrator or go thru file copy prompting.
WLM opens and sync's again.
MS is just plain asleep at the wheel to keep these files intact.
At least I can receive advanced builds and fix WLM.
I had a further issue the other day when emails arrived in outlook .com via the browser , but would neither send or arrive in WLM . One day later the emails suddenly appeared ! How can we trust WLM?
It seems the answer to the question I was going to ask is….there is no answer.
I’ve had several of the problems listed and discussed – nothing in sent box, delays in mail moving from outbox, unable to send or receive (0x80004005), all seemingly as a result of the update designed to keep WLM 2012 active on my Windows 7 machine. Where my problem differs is in that I have four accounts, two are mine and two I administer for my (even less techie) partner. Three still work fine, but my “default” account is dead! All the settings seem the same so why should only one fail? Or am I tempting fate and the others will crash next week? Despite uninstalling and reinstalling Windows Essentials the problem actually got worse (developing from nothing in the sent box to not able to receive or send etc). It seems the best thing is to carry on with the three on WLM and I’ll use Outlook until some sort of fix is found. I will certainly not be installing any patch or update until I’be seen the feedback from users!
Apart from the issues and workarounds already listed below:
I am now failing to receiving my emails in WLM yet if I log into outlook .com I can see 6 waiting to be read which have been sat there 5 hrs !
Are we going to ever get answers or a proper fix since the updates messed it up!