Se uma conta de domínio for usada: O MSSQL pode iniciar se a conectividade com os controladores de domínio estiver temporariamente inativa?
No, even if you get MSSQL to start it uses authentication more than just when starting. If you are worried about downtime for patching dcs, etc, I would recommend investing in more than a single DC or a more robust AD infrastructure.
Cada instância do DBMS em cada servidor deve ter uma conta separada, ou faz sentido usar uma conta de domínio "MSSQL" específica em todas as instalações MSSQL na organização?
Each server should have its own account. You can go with a single account without issues but keep in mind this limits you in terms of flexibility in distinction between the servers using this account
Se contas separadas forem usadas para cada instância em cada servidor: Faz sentido criar um grupo especial de segurança MSSQL no domínio e colocar todas as contas de serviço MSSQL nesse grupo, talvez para facilitar a replicação, etc?
Groups for the accounts is good, I believe when I set it up it wasn't necessary for a group, but wouldn't hurt.
Existe uma convenção de nomenclatura comum e geralmente aceita para contas de serviços MSSQL?
This should be in alignment with your organizations naming standards. This depends on the size of your org, and sometimes on if you are clustering. Keep in mind only use something like sqlserver or svcsql if you are 100% sure there will not need to be distinction between these. A better idea might be to use the host or cluster name. svcsqlnode1, svcsqlcluster1. These are general examples and always try and align with the standard naming conventions.