1). If the "notched button" will not move when you try to turn it in a clockwise direction, then the counter wheel has seized on its support shaft; you should not try to force it as you might cause serious damage to the frame counting mechanism. The reason is most probably that the lubricating grease used by Hasselblad has become "glue like" through drying out over the years; the magazine will need taking apart, cleaning, lubricating and reassembling. I use watchmaker's oil rather than grease when I service my magazines.
2). Cassettes can be bought on ebay for example.
3) Out of date film may be purchased also on ebay.
4). Fresh film is available from Maco direct and via the Ilford ULF 2017 run. Film Ferrania have said they will make it available at some time in the (far?) future.
Where are you located?
Hope this helps you a little.