HI
For Question 1 and For Question1A
since server essential 2019 and previous version need to be root DC with Full FSMO,If you purchasing 3 product keys,you need to run 3 server essentials in different domain.So I think this is not the option for you.
for Question1B
I have seen one user install web server on the physical server essentials 2012r2,so i think it can come true to install web server on server essentials w2019.
for other Question
And then if you are allowed to have 1 free VM, would it be better to run the email server on the VM, or the web server?
yes,if you install only hyper-v role on physical server essentials 2019 ,you are allowed to have 1 free VM with server essentials 2019 version,since server essentials 2019 has been removed server essentials experience role ,i think running web server on server essentials 2019 can be completed easier than previous version.since web server need to install SQL server and we have only one public ip,I think we can use Microsoft 365 Business for office app and email so that we don't need to install exchange server on server essential.
Hyper-V support and licencing in Windows Server Essentials 2019
https://social.technet.microsoft.com/Forums/en-US/1515015a-55d7-411e-a181-41e4dc97b807/hyperv-support-and-licencing-in-windows-server-essentials-2019?forum=winserverhyperv
What's New in Windows Server 2019 Essentials
https://learn.microsoft.com/en-us/windows-server-essentials/get-started/what-s-new-19
SQL Server 2019: Hardware and software requirements
https://learn.microsoft.com/en-us/sql/sql-server/install/hardware-and-software-requirements-for-installing-sql-server-ver15?view=sql-server-ver15
Microsoft 365 | Business
https://www.microsoft.com/en-us/microsoft-365/business