We reviewed your posts (Thanks for such detailed description of the problem!) and think that you have next problem:
1) Your ordered certificate under Administrator account (your private key was generated and stored for this user)
2) You have tried to install it under Normal User account (actually any different user to Admin) and you received correct error.
The possible solution for your problem:
1) Login as Admininstartor
2) Click the link to collect the certificate.
3) Open this link in IE and certificate should be installed.
4) Export certificate from Administrator Personal store to file.
5) Import this certificate in Any User account you wish to
I hope this answer will help to resolve your issues.
Feel free to ask any questions on CSE.
P.S. We'll review issue with installation crash with "Returnil" and "MakeMeAdmin" run.
Thanks for the bug report.