You don't want to update the node tables directly. You would miss out on the hook implementations. You need to create a node object and use node_save() to do it. You could manage this by creating a custom module to push and pull the changes to an external DB for the ms access entries via a hook_cron implementation. There may be a module already to do this but I don't know.
What are the hook implementations you mention specifically??
If the data tables are all updated correctly will drupal not see it as a node?
As mentioned we aren't programmers so this seemed like the easier option..
My concern with trying to write something to keep two sets of data "syncronised" is managing the whole syncronisation process and issues with changes happening on both sides causing a "split brain" scenario.. If both access interfaces are editing the same data this is reduced.. No different to why MySQL replication is not often attempted as master-master because the sync is not easy to do reliably..