I have thought a lot about titles and summaries, but haven't yet decided how to proceed. There are too many special cases.
The top-level heading in a Mitra post is a title, and shouldn't be used as a summary. There is a different attribute for titles in ActivityPub, name. However, If I extract the <h1> and federate it as a name, other Mitra instances will display two titles.
A summary, ideally, should not be displayed anywhere, because it is a summary of content. It's redundant by design. But Mastodon uses it as a "content warning", so Mitra has to insert it into the post to avoid losing important information.
If Pleroma uses summary for "subject", that makes things even more complicated.
There is probably a way to make it all work, but I don't quite understand your use case. What exactly you're trying to implement, a content warning?