>My hope is/was that, if the 5.1 upgrade fails, I could simply failback to a clone of my VMware vCenter Server made prior to the attempted upgrade (it is 4.1). Is there more than this or more to deal with?
Restore of the database is recommended as a clone is not a proper way to backup a database.
>I plan on making a clone of the vm, I also have a Veeam backup job of the VM that run VMware vCenter Server. Finally, I will make a backup of the SQL Server Express database itself.
Good 
>Do you mean in the vCenter Server 5.1 install? I cannot find an "alternate name field" in 4.1 vCenter Server via the client. Here is what my SSL setting currently look like under Administration > vCenter Server >Setting > SSL Setting:
I mean the certificate in c:\ProgramData\VMware\VirtualCenter Server\SSL
>Also, I assumed the, "vCenter Host Agent Pre-Upgrade Checker" would alert about things such as this, or will it?
No
> Again, will the "vCenter Host Agent Pre-Upgrade Checker" do this?
No.
> I'm following diligently the Upgrade Guide. Is that good enough? Is there a good forum post or blog post on this procedure or is it just not _that_ big of a deal?
This should be enough
> take this to be recommended install/upgrade procedure, in this order?
You can install the Web Client directly after SSO but the order SSO -> Inventory Service -> vCenter Server is mandatory
> Thx much for the help!
You are welcome.