I'll go straight to your issues:
1. I've tried to reproduce the different behavior of fields displaying between the header and the main body of a document, but to no avail. I've created such a document with RadRichTextBox and import it back there and everything works as expected. I've tried the same with MS Word and everything looks OK again. You can find the documents attached.
2. I can confirm there is difference between the tab stops which are defined in a paragraph which is in header/footer in MS Word and those in RadRichTextBox. Actually, in RadRichTextBox, there aren't such tab stops declared and thus when a tab is inserted, the position is moved with the default tab stop width. I've added this feature request in our backlog and here
is the related item in our feedback portal for which you could vote and subscribe.
About your last question - performing mail merge
is not an undoable operation and thus you cannot perform undo on it.
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