An i-140 can be withdrawn after or before 6 months, it's totally upto the employer. Infact the sponsoring company is obligated to withdraw the i-140 petition as it's a reporting requirement to the USCIS. I think we need to clearly distinguish between withdrawing and revoking. Revocation only happens due to fraud, misrepresentation, or material error as I mentioned earlier(AC-21 rule) which in general doesn't apply to most people and only in that case is the PD lost forever.
-- If you read my post clearly, it distinguishes between withdrawal and revocation. It was common practice of USCIS to revoke the I-140 after employer requested to withdraw it (without any fraud, misrepresentation or error). Obama era regulation gave protection from such revocation only if 6 months have passed after approval. Even then I recently read some cases whose I-140 got revoked after 6 months of approval. In that case if 6 months have passed then you at least have some ground for appeal. No such protection if 6 months have not passed. Hence, one should wait 6 months after approval to quit the petitioner.
When it comes to PD, yes it's going to take forever to get a GC but suppose there is a 1% chance four or five years down the line where there is a legislation passed to provide GCs to all those that have a PD + a US Masters degree(just giving an example) then it would be regretful if you can't take advantage of such a provision specially if your GC process was started. Chances are slim but why not keep options open?
-- Yes, quitting after 6 months and keeping the I-140 valid gives you such option (even though slim).
As far as H1B goes, if you're migrating to Canada, your employer would've withdrawn your i140 petition so a new i140 petition would be required although you can port your old PD. So not sure how H1B extensions are applicable if you've resettled in Canada.
-- If the I-140 remains valid, another employer can file an H1 transfer and the person can return to US immediately. Yes, another I-140 will be needed, but that will be needed anyway. Employers are more willing to quickly transfer H1B rather than filing a cap-subject petition. If you have not used 6 years of H1, then you don't need the I-140 for transferring. However, it will still be needed for PD porting.