Are there tools available for importing an Excel file to a SharePoint list so that the various column types would be supported, and that the importing data would be synchronized based on an a value in an id field, for example? I am interested on a customizable solution ie preferrably not a commercial app with non-public APIs.
Have you looked at this option: (Requires an Excel plugin)
Publishing and Synchronizing Excel 2007 Tables to SharePoint Lists
It supports having Date and Text columns as per my memory. More column types may be supported, but I am not sure.
Option 1: A free solution that requires that you create the SharePoint list in Excel, requires an Excel plug-in (released by Microsoft) and requires that you save in the 2003 xls format to retain the sync.
Option 2: A very inexpensive solution similar to the first but does not require you to create the SharePoint list in Excel first. Has an API.
Option 3: A wildly effective solution with nearly no limitations. Unfortunately, it is comparatively very expensive. Has an API.