The problem is that we'd like to ask a lot more interview style
questions beforehand to inform the development of our user personas
however this is starting to feel like a different exercise and I'm
wondering wether it's a good idea to combine design research and
usability testing in the same session?
It's not uncommon to ask the user pre and post interview questions to both establish an audience baseline and qualitative feedback on system usability. However, these are more related to the experience of the test rather than exploring facets of different users of your system. It's recommended that when doing persona interviews, patterns begin to emerge after 25-30 users. You probably don't to need run a usability test on that many users, and while you probably will get some valuable persona information from 5-6 users of the usability test, it won't be enough to look for patterns to fuel a cluster analysis.
Another challenge will be managing the users time and attention. For each user you'll have realistically about 30-45 minutes of quality attention from the user. A persona interview is typically more conversational in nature and could quickly and easily take up the hour scheduled for the usability test. A usability test will be driven by the task at hand. Some participant may have a problem efficiently switching modes of thought. That said, I think these efforts should be split so that you can get the most out of each effort. Whatever you decide to do consider having a second person in the room with you facilitating the note taking. As moderator, it can be challenging to keep the interview/test moving while observing and taking notes. The second person will also serve to catch additional insights that you may have otherwise missed.
Another concern is that the production team want to conduct the
usability tests themselves having been provided with a script because
of concerns about impartiality. Is this a better alternative to having
the same UX team that will be working on the website conducting the
This depends on whether you can remain impartial and which team has more training/insight in how to conduct a usability test. Impartiality is of upmost importance. You don't want to be leading the user or influencing the results in any way. In my experience UX teams have greater experience/interest in conducting usability testing but are challenged, understandably, by internal biases. To quote a passage from UXMatters.
The fact that you’ve taken a stand and created a design in the first
place means you’ve articulated your design hypothesis and instantiated
your hypothesis in the form and function of your design. It is going
to be difficult to keep yourself from wanting to confirm your design
hypothesis, because you’re hardwired to preferentially seek out
confirming rather contradictory evidence. Even though you’re able to
criticize your own designs and recognize that a fundamentally sound
design needs some adjustment, confirmatory bias makes it hard for you
to realize that your design is the wrong approach entirely.
In a perfect world we would have a third party test our designs, or a UX researcher embedded in the UX team for this very purpose. However, with organizations downsizing UX resources we are often faced with both the need to design and validate. In your case you have a team who may have less internal bias willing to perform the testing. If you feel that can remain unbiased, and conduct the interview effectively I'd say go for it. You can alway double as the note taker and participate in the interview observations/feedback.