I’d be pretty sure that historic species lists for obscure locations have a lot of research value. Whether iNat is a good place to put them depends somewhat on how much energy you have and how you might want to use them.
The iNat staff do emphasize that iNat is about building community and engagement around observing nature. Obviously, your father is no longer here to be part of that community, and without photo or other documentary evidence, these historical observations won’t be listed as formal “Research Grade” data.
But there’s still substantial value that might come from adding observations like these. Firstly, researchers and other iNat users have access to query and download any records not just RG ones. So, someone researching a particular species would very likely include “Casual” observations in their starting data set and then make their own choices from there. One important effect of RG status is that only those observations are candidates for automatic inclusion in external datasets, such as GBIF.
From a different perspective, adding historical records may provide valuable context for your own present day usage, such as to decide where to search for the continued presence of particular organisms. It would allow you to use iNat features such as maps and phenology charts to combine and compare these data sets.
Because there are no images involved, you’re likely not adding a great deal to iNat’s costs, except for some extra indexing. I’d give some thought as to whether you want to add these observations under your existing @tracc account or create a specific account in your father’s name. Separate accounts would perhaps clarify who made the observation, but you would be adding complexity by having an additional account to monitor.
Also, for taxa whose location are automatically obscured due to conservation issues you would likely find that observations from a second account would have auto-obscured locations. This would make mapping difficult, although you could use a project structure to get around it. If you go with keeping the same account, I’d recommend you add something on your bio to the effect of “Observations prior to 199x were made by … and have been transcribed from species lists compiled for biodiversity surveys.”
Lastly, given that you’re looking at thousands of records, maybe start with a limited subset (one survey?) and see what comes of it.