Normal OPC writes only write the item's value. There is a provision in OPC Data Access 3.0 for writing combinations (value/timestamp/quality), which is what happens when they are all present in the mapping and you perform a mapping Write.

There is a Operations members to the [DAItem] attribute, which defaults to DAItemMappingOperations.All. You can exclude the Timestamp and Quality mapped mebers from writing by specifying (in the [DAItem]) that the Operations should not include writing. It is a bit mapped enum, therefore you can combine the individual operation with a '|' operator.

Note: I will answer all your questions, but due to their number, please allow for some time. I appreciate your interest, though!