A frontend-only implementation suggestion:
Rather then posting a completely new post, post a JSON Obj. as a post (custom JSON cannot be voted on to my knowledge?), structured as such:

{
"ctx": [
[
"post": <STRING: post URL>,
"time": <INT64: timestamp>
] ... // as many as needed
],
"post": <STRING: post URL>,
"time": <INT64: timestamp>,
"com": <STRING: HTML comment || NULL>
}

When sharing a regular post, ctx is null, post is the post's URL, time is sharing time and com is any optional commentary. Beneficiaries e.g. 20% sharer, 80% original author

When sharing a comment, ctx is original post's URL, post is the comment's URL, time is sharing time and com is any optional commentary. Beneficiaries e.g. 20% sharer, 40% comment author, 40% original post author

When sharing a share-post, ctx is a merge of shared post's ctx and post fields, post is shared share-post, time is time of sharing, com is optional commentary. Beneficiaries e.g. 20% sharer, 40% shared share-post author, 40% divided equally between ctx authors

Any other identifier could be also used instead of the URL. (e.g. a partial URI or other identifier)

Each share-post would DISPLAY as ctx at time of sharing (with link to original), post at time of sharing (with link to original), and commentary (HTML).

It's quite a bodge but it's frontend-only.

As always, I'm open to suggestions.
Apologies for brevity, composed on a mobile device.