How to create a commenting system for ECK entities, similar to node comments as exposed field under the body? I have done quite some research to accomplish comments under ECK entities, but all the tried solutions and modules appeared to hamper or be incompatible.

The idea is to create an exposed field for directly entering comments. It should allow for multi-level commenting (replies to comments).

I will update this question with the tried solutions.
If you have actually tried or implemented a commenting solution for ECK entities yourself, please post your answer.

If you have suggestions but have not tested it yourself, please post it as a comment to this question, and I will try your suggestion.



Option: module Comments Entity.
I can't get beyond a bug I posted in the issue about "Cannot save comments".
As an alternative to the Comments Entity module, and the bug you reported about it, you may want to give the Reply module a try. Some more details about this module, from its project page:

Reply is a D7 answer to entity comments.
Reply is basically a fieldable entity so you can create a 'review', 'comment', 'forum post' or whatever you like by creating bundles and attaching fields to them. Such bundles can be attached to any fieldable entity by 'reply field'.
You can set access for reading and writing, position the form to the same page as entity or to a custom page, allow or deny replying to replies(= no hierarchy), choose threaded or flatted list. Such settings can be set for bundle, field, instance and entity with build-in inheritance which makes if a four level hierarchy settings system.
Most common usage of this module is comment replacement and reviews.

Even though this module only has an alfa release for D7 (as compared to a beta release for the Comments Entity module), the issue queue statistics seem to be more healthy:

Reply:  66 open, 166 total (about 40% open issues).
Comments Entity:  10 open, 10 total (= 100% open issues).