Thank you Kevin for those videos. I have some back troubles as well so they will definitely benefit me as well
I am currently also working on sitting cross legged while still having a straight spine (can't do that yet).
Sadge, have you tried a meditation bench? Or a cusion? You don't have to sit cross legged that way.
There's no technical reason why a chair would not be a good idea, although it may depend on your mind's associations with chairs. If your mind goes instinctively into relaxation mode, it may be a difficult 'environment' to meditate in (not 'bad', just a higher difficulty level). And, as I have been taught, whenever it is physically possible for you to do so, support your own spine. Do not lean back. This helps your alertness etc, which is also touched on in those videos.
Either way, good luck finding a comfortable posture