I think you basically have a people problem not a technical problem. Anyone with sufficient access and sufficient skills to configure/fix servers for you, will be able to avoid/disable/obfuscate monitoring tools.
I believe the solution to your issue will be basically building trust in a company/freelancer who can help you on an ad-hoc basis.
in the past I've solved similar issues (basically, how do they trust someone they just "met" on HN with root/sudo access) with clients by doing basic diagnosis via a non privileged account, and asking them to run commands/scripts as required (either for deeper diagnosis or a fix). Obviously I explain the purpose/goal of the things I ask them to run, and I'd invite them to get a 2nd opinion about the purpose/safety of said commands.
<shameless plug>
If you're interested in trying something like I described, drop me a line (see my profile).
Edit: wtf typo.
