These medieval portative organs, so extensively used during the 14th and 15th centuries ..... contains nine pipes.

Some of them contain more than that, and often with two much longer ones which had to be used as drones. (Catalina Vicens has a lot of illustrations of them on her FB pages). From the lengths, you had a choice of two drone pitches a tone apart, as with the "Durer" style of bagpipe.

A mode is not constrained to start on any particular pitch, it is just a sequence of intervals, like our modern major and minor scales.

Urban legend. No they WERE fixed in pitch, and still are in may modal idioms. They were far more than just a sequence of intervals.


So it may be (tho' others may well know better) that every mode would start on the lowest note of the keyboard, and these primitive instrunents had to be retuned each time the mode changed, thus keeping the pitch at a comfortable level for the singers

A portative organ is not retunable like that. And they were most commonly used as a solo instrument.