How can Springer and Wiley put a 12 month embargo on posting post-review revisions to the arXiv and similar?
The copyright agreement does not require that you leave your mistakes uncorrected, it simply requires you to make corrections in a particular way. The simplest method, if you discover mistakes in the published version, is to distribute some note of correction that specifically addresses your original error, which can be done without violating the copyright agreement. The copyright agreement may allow you to post post-review versions of the paper, since the agreement may contain a clause that "Prior versions of the Contribution published on non-commercial pre-print servers like ArXiv/CoRR and HAL can remain on these servers and/or can be updated with Author’s accepted version". There is always a clause where the author warranties that the work has not been previously published (which includes online distribution via ArXiv), and this is why they need to expressly say that you are also permitted to post the submitted version.

There are two notions of "enforceability" relevant to your agreement with the publisher. The stronger one involves litigation, where the publisher sues you for damages (when you breach a contract). You should hire a Swiss lawyer to get advice on whether Springer is likely to prevail in court. The other notion is "having negative consequences". A simple negative consequence would be that Springer refuses to publish any more of your work, if you flagrantly violate the terms of the agreement. Whether or not your breach involves copyright violation depends on whether or not you give them a license to publish, versus transfer copyright. (In interpreting the Lecture Notes in CS agreement, you would want to consult the Swiss attorney for a precise interpretation of granting and assigning the exclusive, sole right to copy).

Your comment about the review process is tangential, and suggests that you are unaware of review protocol, You ask "Why one should review for free for a journal that won't let the author share the results of your review with the community in as timely a fashion as possible". Reviews are not only anonymous, but also privileged communication between the reviewer and the editor (often -- though not always -- shared verbatim with the author). So you should not distribute reviewer comments, unless that is expressly permitted by the journal.