Hi Tedsec and welcome,
I'm not sure that SharePoint 2010 would help you a whole lot. InfoPath to SQL is a major gap for SharePoint. BCS is still not a good solution because it creates a tight binding. The reality is that forms change a lot and there needs to be an abstraction layer (Web service) to handle the mapping when things change.
Disclaimer: we use SQL here for all of our InfoPath forms work. SQL Express is free and has more functionality than Access.
Storing data in SharePoint is effectively storing it in SQL since SharePoint uses SQL behind the scenes for all data storage. SharePoint lists and form libraries are represented as tables in SQL and therefore they don't support repeating structures. In other words, you can't submit a repeating group in InfoPath to a SharePoint list nor can you promote those values to a SharePoint form library.
Regarding your database submit ... back in 2004 we created a Web service called DBXL that maps repeating structures to SQL after-the-fact. In other words, you can take a stock InfoPath form with an XML data source and create a mapping to SQL. No code required. As you can imagine, this has been very popular (MS bought 8 licenses...) and we continue to sell this "accelerator" even now, 7 years later, because nobody has bothered to come up with an alternate solution (boggles my mind). The accelerator is inexpensive and can be used for all of your forms. If you aren't completely satisfied, full money-back guaranteed, etc. Lots of happy people use this every day and while it is middleware (impossible to effectively market) I'd be happy to show you a quick demo or you can go to our Web site and watch the videos, etc. Bottom line: it's way cheaper than trying to do this in code yourself and it will work for all forms down-the-road.
Now, I'm sure I don't understand your scenario well because I am human... so, let's say I am totally off and you could care less about submitting to a database and what you are really concered with is the query of the SharePoint lists ... if so, maybe you need to copy the data from your secondary data source to a properly repeating main data source. That requires code. Maybe you are an avid coder but best practice is not to write code for every form. So, four years ago, we developed this plugin called qRules which is also a free download for a trial version, which basically includes all of the features in InfoPath that people regularly request but for which code is required. One of the most requests is copying data from one table to another. You'll need to do that to get it in a format that is propery structured for submit to Access. qRules is a simpler thing since it's client-side only (no server install) and it's even cheaper plus you get Qdabra's world class support and you can feel good about returning some love to the sponsors of this forum (this is a total loss leader for us).
Or, you can just pay someone hourly to investigate, but my #1 hunch is that you have a data source shape issue on submit and my #2 admonition is that you should use a Web service for this kind of thing.
Best of luck!