1) There is recurrent discussion concerning the terms "blank" and "empty", sometimes including additional ones.
2) To get it clear I need an example .ods demonstrating your observations. I will upvote your question sufficiently nourishing your karma.
3) I never got an error with =ISBLANK(CellReference) as you describe it. Please add information about your OS and versions of OS and Calc.
4) Concerning =TYPE(AnyValue) you should study not only the help text but also the ODF-specification . If you then decide to regard TYPE() as a deprecated function and to no longer use it, that might be a good idea. This related with the issues coming about because of the treatment of "logical" or "Boolean" by standard spreadsheets. "It's a mess, not a bug" (Lupp).
5) "T(x) produces a blank ...". Again the terms. You are using "blank" now explicitly for the zero-character-text. TYPE() will correctly return the code 2 for it.
6) " I don't get the logic of this." You shouldn't expect too much of logic in the details of Office software. The software is just the current state of a history. A truly consistent specification and implementation has never occurred. marketing and "compatibiity" were the ruling concepts, not clarity or logic. ODF tries to specify a persistent representation of documents and some functional aspects reliably. It's stuffed with history and compatibility issues nevertheless.
