The Set-PowerCLIConfiguration cmdlet has 3 scopes (Scope parameter).
If you want to change it for the current session, you do
Set-PowerCLIConfiguration -defaultviservermode Single -Scope Session
But I find it strange that you only see 1 connection in $defaultviservers.
Let's first set the config to single and then do the Get-Datastore again to see if it changed the result.
Update: which PowerCLI build are you using ? Do a
Get-PowerCLIVersion