Defender definition updates

Unfortunately not direct, but you could use an Advanced Hunting Query: https://github.com/microsoft/Microsoft-365-Defender-Hunting-Queries/blob/master/General%20queries/MD%20AV%20Signature%20and%20Platform%20Version.md

If you are Using Microsoft Endpoint Manager (Itune) or SCCM, you can check for the Definition- and Platform Version there: https://deviceadvice.io/2020/12/07/manage-and-report-on-defender-antivirus-signature-update-versions-through-microsoft-endpoint-manager/

You could also build something yourself using Powershell Commandlets (Get-MpComputerStatus): https://docs.microsoft.com/en-us/powershell/module/defender/?view=windowsserver2019-ps

About your Question Nr. 2: Unfortunately, the best I know is that you read alle the available stuff in Microsoft Docs around Defender for Endpoint.

GReat Ressources are:
https://techcommunity.microsoft.com/t5/microsoft-defender-for-endpoint/become-a-microsoft-defender-for-endpoint-ninja/ba-p/1515647
https://github.com/alexverboon/MDATP#microsoft-blog-posts-on-microsoft-advanced-threat-protection