Siobhan, my background is in meat processing in Canada (RTE and RTC)
Yes, micro for this to prove the log reduction should go to an outside lab (unless you have your own in house certified lab) They will need samples from the same batch pre and post cook (i.e. raw to check existing levels and fully processed to validate log reduction)
The validation for the meat can be done in house rather easily for time/temperature. Place meat in ALL possible locations on each belt for each oven, mark the piece so you don't get mixed up and check the wrong one. And no, you won't be able to do this in a single day, it may take a month to complete. (i am wondering why it hasn't been done prior to your arrival there however)
You will need to measure by weight, thickness etc for EACH piece. Take the temperature prior (you may have a huge variance here) and time each piece precisely. Then record the internal temperature in the thickest spot (alternatively purchase data loggers that can withstand the over temperature, some are even wifi enabled and will automatically dump the data for you). Repeat this procedure under all conditions (belt speed to high, oven temp 5C lower than normal (worst possible scenario).
Then once all of that is done (again, this is not a quick process nor should it be) you can extrapolate the information from the data taking all parameters into consideration. Try not to make any assumptions about the process until the data is complete. If any of the pieces fail, you will have to repeat the series and tighten up the working parameters.
I gather from your post that there is not an existing HACCP plan, this process is slow and arduous, plan well and execute slowly. If the process needs to change based on the documentation you gather, then it needs changed. Its better than the alternative.