Constant errors stating the following were the main contention:
New-SCVMTemplate : VMM is unable to process one or more of the provided cmdlet parameters. (Error ID: 1600)
Needless to say, this is a bit non-specific! After a lot of frantic searching of blog posts and whatnot, and complete inability to find a relevant fix, I decided to scrape through the code and remove parameters one by one. Eventually, the Template parameter seemed to show up as a potential problem.
Inspecting the template I was intending to use, I noticed something - there was no OS Configuration section, therefore no ability to pass parameters such as Domain or DomainJoinCredential. Fixing this resolved the above error.
Now, on to simultaneous deployments!