Hello Edie,
Looking through your case notes, the first question I have is whether you're installing Business Portal 5.1 onto a site collection that has alternate mapping on it and/or URL re-direction?
The reason I ask is because normally the site collection URL will be something like http://servername:port#/
When you install Business Portal, it simply uses this URL: http://servername:port#/BP
You mentioned you were seeing an URL of http://myserver/BusinessPortal/, which makes me think you may be installing Business Portal onto a site on top of a site collection and/or have AAM setup on the site collection and this can actually cause issues with the install of Business Portal.
What we recommend, if you haven't already done so, is to use a site collection of the default http://servername:port#/ to install Business Portal onto and then test with the http://servername:port#/BP URL.
Once the installation is completed successfully and you've tested and verified everything works, then you can add any alternate access mapping, URL re-direction and/or SSL to the site collection/URL as you need.
As for the message about only being able to save, not submit requisitions if you're not in the actual approval hierarchy setup for the company you're in, for Requisition Management, that is normal, but you shouldn't be seeing the other errors after that.
Since you've been working on this for about a week, I would suggest creating a support case so we could look at it closer with you.
Thank you,