Powershell Special characters / Automating build - PSRemoting - powershell

Question

I am using Teamcity and Powershell to automate few builds. Teamcity runs under lets say account A. There are few executable that needs to be run under account B. Account B has access to DB (windows auth integrated security access) and needs to go and create few tables on install. Passwords for account B is saved in the text file. I am having few issues with this.
1) Account B can have any special characters. I am reading XML file (as this file can be saved in a location where only Account A has access) and new up PSCredential to do the PSRemoting. I am using something like
$secpasswd = ConvertTo-SecureString $passwordFromXMLFile -AsPlainText -Force
$mycreds = New-Object System.Management.Automation.PSCredential ("username", $secpasswd)
$PasswordFromXMLFile can have any special characters. How should I deal with this?
2) When I try to run the executable in context of account B (after I somehow escaped and connected using this account), I am seeing the executable runs under anonymous account instead of domainname\B account. Is there some setting that I need to make sure it runs under account B.
My code looks something like - this is not the exact code I am just extracting some relevant portion.
$secpasswd = ConvertTo-SecureString $passwordFromXMLFile -AsPlainText -Force
$mycreds = New-Object System.Management.Automation.PSCredential ("B", $secpasswd)
$session = New-PSSession -ComputerName $serverName -Credential $mycreds
I see the session connects fine. to the computer
$scriptblock = {
Start-Process $exeName $arguments - wait
}
invoke-command -session $session -scriptblock $scriptblock
Thoughts? Start-Process runs but it executes on anonymous authority instead of account B.