1) The only embedded database that supports concurrent editing is VistaDB (at least I had no success to find any other option)
VistaDB official site -> take a look at Pros/Cons section! - (..) Embedded databases typically lock the entire database per insert operation, we do not. We provide table and row level locking.
Good thing about VistaDB - it supports Entity Framework Code-first!
Regarding price - there is a discount for micoISVs (450$ + 125$ upgrades for one year)
2) If you know that concurrent writes to database will be very rare - then you can choose SQLite, which is a free embedded database - because SQLite allows concurrent reads, but only one connection when writing to database. Did not find any information about EF support.
3) If you will have only very few users then SQL Server Compact could be the best option - it has support for Entity Framework.
