As for the bonus question: Can we combine them and just add one after the other?

We've been thinking about combining, but then @Jonas brought up the possibility to abuse the summary and "hide" some bad edits. Given comment https://phabricator.wikimedia.org/T173744#3889897, it seems we're fine with going this path then.
All clear, will start working on this task!

Change 405316 had a related patch set uploaded (by WMDE-leszek; owner: WMDE-leszek):
[mediawiki/extensions/WikibaseLexeme@master] Set summary when editing form elements with the UI-dedicated API.

https://gerrit.wikimedia.org/r/405316

From the discussion I had with @Lydia_Pintscher on Wednesday:

• In our discussion we confirmed this: The user is able to edit as many grammatical features and representations as he wants the same time. One click on the save button should trigger one API request, and result in one history entry with one summary line.
• Concatenating multiple atomic localizable summaries is a new concept for Wikibase and wikidata.org. (Even if is not a new concept for MediaWiki core.)
• In the discussion we concluded we don't want to introduce this new concept (with all the consequences that follow, including UX considerations) right now. Possibly later, but not for this story.
• Summary lines should be precise. Ideally the line should allow to understand what an edit is about without looking at the diff.
• But this does not mean that all summary lines must always repeat all details from the edits they describe, no matter how complex the edit is. That's still the job of the diff.
• Lydia estimated that use cases where only one element is touched (e.g. only one grammatical feature, or only one representation) are the majority. That's what Lydia cares about. If the majority of edits does have precise summaries, Lydia is happy. The remaining use cases that produce generic summary lines can be improved later, if needed.

The summary lines needed right now are:

• Adding one or more grammatical features: "Added {{PLURAL:grammatical feature|grammatical features}} of L1-F1: …, …"
• Removing one or more grammatical features: "Removed {{PLURAL:grammatical feature|grammatical features}} of L1-F1: …, …"
• Adding one or more representations: "Added {{PLURAL:representation|representations}} of L1-F1: en: …, de: …"
• Removing one or more representations: "Removed {{PLURAL:representation|representations}} of L1-F1: en: …, de: …"
• Editing one or more representations: "Edited {{PLURAL:representation|representations}} of L1-F1: en: …, de: …"
• A generic summary for all other types of edits.

Two additional summaries can be considered (@Lydia_Pintscher, please decide):

• When grammatical features are added and remove the same time, a generic summary can say "Changed {{PLURAL:grammatical feature|grammatical features}} of L1-F1: …, …". The list does not show what's removed or added, but all elements that are there after the edit.
• When representations are added, remove, and/or changed the same time, a generic summary can say "Changed {{PLURAL:representation|representations}} of L1-F1: en: …, de: …". The list does not show what's removed or added, but all elements that are there after the edit. Note: This is a new suggestion currently not listed in the tasks description. The difference between "Edited representations" and "Changed representations" is that "Edited" only lists elements that are actually touched in an edit, while "Changed" must list all.

Change 409380 had a related patch set uploaded (by Thiemo Kreuz (WMDE); owner: Thiemo Kreuz (WMDE)):
[mediawiki/extensions/WikibaseLexeme@master] Fixups to Form edit summary classes and related

https://gerrit.wikimedia.org/r/409380

Change 405316 merged by jenkins-bot:
[mediawiki/extensions/WikibaseLexeme@master] Set summary when editing form elements with the UI-dedicated API.

https://gerrit.wikimedia.org/r/405316

Change 409380 merged by jenkins-bot:
[mediawiki/extensions/WikibaseLexeme@master] Fixups to Form edit summary classes and related

https://gerrit.wikimedia.org/r/409380