Need SEO professional for re-indexing website

To make this very clear at the beginning: Please, don't contact me, if you are not a SEO pro. I am not a rookie myself and know a thing or two about SEO. Now, here's the problem: In July my customer wanted to switch her pretty good ranking old domain to a brandnew domain. We advised her not to do this. However, customer is king, and we performed the transfer. Everything went great. Within two weeks, the new domain was almost as good as the old one (according to ahrefs metrics). I started a domain transfer in the Google search console and de-indexed all URLs of the old domain. Redirections etc. were all perfect. Two months later, my customer decided to re-transfer the new domain to the old domain. She understood that the first transfer was a mistake. Pretty bad situation. We told her that this was an enormous risk. Nobody could know what Google what do. We started the re-transfer and cancelled the transfer in the Search Console. Even hardcore SEO pros often do not know this feature. But it is there. Again, redirections etc. were all perfect with no errors or issues whatsoever. Sitemaps were, of course, submitted in the Search Console. Site search was ok, the live domain was present in the Google index. Some weeks later we found out that 180 URls of the inactive domain were still indexed. So, I removed those URLs in the Search Console. Within a day or two the live domain lost almost all URLs in the index. Since 3 weeks only 10 URLs can be found with site search. However an individual site search (site:domain/page) shows that the URLs ARE indexed. Example: site:[login to view URL] shows 10 URLs site:[login to view URL] shows that the URL is indexed, but it does not show up in the site search for the entire site. We need somebody who can fix this. Plese note, this is NOT about content SEO or technical SEO per se. It's about this specific problem.