Will post an answer of my own as well for completeness sake. I sent the same question to CPF directly, here's what they said:
Yes, you are right in saying that your Singapore Permanent Resident
(SPR) status will be revoked when you are no longer residing in
Singapore for a substantial amount of time.
We would like to clarify that you can ...
Yes, you can keep your savings in CPF essentially forever even if you leave Singapore. (Or that's the rule at the moment, anyway; you have no control over CPF investments and are subject to the whims of the Singapore government here.) In fact, the process of getting your CPF money out is rather bureaucratic and will definitely not "just happen" on its own.
CPF will not cancel your account or instruct the providers of your Special Account unless you specifically request them to do so.
If you are not Malaysian and residing in West Malaysia and then even so, withdrawal should be straight forward.
I suggest you email the the CPF board and clarify: email@example.com
Is there is a chance I can get the permit renewed if I Volunteer to contribute to the CPF? If so, what are my chances?
I don't think volunteering to contribute to CPF when not working is even possible; this is intended mostly for self-employed people.
If my re-entry permit renewal gets rejected, is it mandatory to renounce my PR?
No. Your REP lets you ...