If you are TOTALLY desperate, you can try the following PowerShell script that saved me a few times.
That PowerShell script generates a ".bat" file, that you can run for completely uninstalling SQLServer. Please do a THOROUGH review of what's in that .bat file before executing it! Possibly, execute things one step at a time:
# Be careful with that axe, Eugene.
#
# Generate a bat file named 'uninstallSQL.bat' to completely uninstall SQL Server from a machine.
# Content of the .bat file MUST be reviewed before execution, in order to REMOVE things you want
# to keep.
Get-ChildItem HKLM:\SOFTWARE\Microsoft\Windows\CurrentVersion\Uninstall |
select @{Name='Guid';Expression={$_.PSChildName}}, @{Name='Disp';Expression={($_.GetValue("DisplayName"))}} |
where-object {$_.Disp -ilike "*SQL*"} |
where-object {$_.Guid -like '{*'} |
% {"rem " + $_.Disp; 'msiexec.exe /x "' + $_.Guid + '" /passive'; ''} > uninstallSQL.bat