The string looks the same (has the 12.0) in BIDS. Whenever I have the 32 bit driver installed, the SSIS package in BIDS validates just fine. Whenever I have the 64 bit installed (and 32 bit uninstalled, since they can't be installed side by side) it fails.

It appears to only recognize Microsoft Access Database Engine 2010 Redistributable 32 bit as version 12. Any manual editing of the connection string still fails with 32 bit mode. The following are not recognized at all:

Provider=Microsoft.ACE.OLEDB.12.0;Data Source=<filepath>;Extended Properties="Excel 14.0;HDR=YES";

Provider=Microsoft.ACE.OLEDB.14.0;Data Source=<filepath>;Extended Properties="Excel 12.0;HDR=YES";

Provider=Microsoft.ACE.OLEDB.14.0;Data Source=<filepath>;Extended Properties="Excel 14.0;HDR=YES";