is there a quality Events calendar plugin that supports front-end submissions from the public, that is known to work well with Toolset/Layouts?
Honestly, not that I know of, but that does also not mean I would know of some that completely fail together with Toolset.
The main issue with such plugins is that they add a feature to WordPress which was and is not really intended by WordPress itself.
Toolset does a bit of the same, sometimes (Example, ShortCodes nesting...)
In those cases it is very often needed to store and handle data in "specific" ways, which then results in this or that compatibility issue, because the other party is suddenly not able to deal with this "specific" kind of data.
Makes sense?
Hence, I cannot name a plugin that granted works with Toolset, as it would require a very complex sequence of Tests.
Related to the specific issues you mention:
- the error https://toolset.com/forums/topic/wp-router-placeholder-page/ is very well explained by Juan in the same ticket
- the issue with "front-end submissions", you might be able to control some of the Post Fields of the plugin (if they store the data in Custom Fields), with CRED; and let the data be submitted by CRED.
But this needs a deeper understanding of the plugin so to know how it stores it's data.
As mentioned, unfortunately event management is not something native to WordPress itself and the plugins offering it, are often handling the data in particular ways.
What I can do is ask our management if we are interested in making some research so to publish a blog post or case study about it.
Would that help?
Maybe we can find a match somewhere.
If you require technical help on the immediate issues you experience, please let me know, we can handle them each in a ticket, maybe we can find solutions for them.
I would need to know steps and errors, so to proceed.
If the plugins are paid, a copy of them (Google Drive Link)
Thanks!