A senior functional consultant could give you the answer to your question in terms of what D365FO allows in such scenarios. I can explain how Docentric pipeline can be involved to email the invoice.
Docentric runs on top of SSRS pipeline. When you start a report, the SSRS pipeline starts, performs security checks, reads and processes the data, and then Docentric intercepts it and continues it’s pipeline. So, if you manage to set/customize D365FO in such a way that the invoice posting is started in intercompany scenario, Docentric will start running as well and be able to email the invoice.