In short: https://docs.jabref.org/advanced/how-to-expand-firstnames for many entries

We have

But we don't have following for all entries:

Maybe: Mass-cleanup of all entries:

Simple implementation:

1. Determine ids (DOI and arxiv)
2. For all entries: Fetch information from doi (and then arxiv) -- Read on at https://devdocs.jabref.org/code-howtos/fetchers.html to learn about the fetchers.
3. For each change: merge entries dialog

Better implementation:

Step 3: Do not show popup. Just take longer string.

Better better implementation:

Step 3: Do not show popup. Use smart heuristics at merging.

I would propose to go with the "Better implementation"