Transactional handling of text files on Windows

How about using NTFS file streams? Write multiple named(numbered/timestamped) streams to the same Filename. Every version could be stored in a different stream but is actually stored in the same "file" or bunch of files, preserving the data and providing a roll-back mechanism...when you reach a point of certainty delete some of the previous streams.

Introduced in NT 4? It covers all versions. Should be crash proof you will always have the previous version/stream plus the original to recover / roll-back to.

Just a late night thought.

http://msdn.microsoft.com/en-gb/library/windows/desktop/aa364404%28v=vs.85%29.aspx

What you are asking for is transactionality, which is not possible without developing yourself the mechanism of a RDBMS database according to your requirements:

"It is very important not to corrupt the content of these files during file IO"

Pickup a DBMS.

See a related post Accessing a single file with multiple threadsHowever my opinion is to use a database like Raven DB for these kind of transactions, Raven DB supports concurrent access to same file as well as supporting batching on multiple operations into a single request. However everything is persisted as JSON documents, not text files. It does support .NET/C# very well, including Javascript and HTML but not Delphi.