How is it possible MST can even work? Example to show

I made a topology, simple one. One MST region, with 2 instances. Idea to make Sw3 root for MSTI 1, and Sw2 root for MSTI 2. See picture

https://i.ibb.co/3zfCxhJ/dnt-work.png

However, since the IST topology is such that the link from Sw1 to Sw2 is blocking Sw1's port, it means that the IST has this port blocked. Thus, MSTI instance 2 cannot work because the IST topology is such that this port is blocked by the IST. Since it can't be designed, I don't see the point of MST. What am I missing?

Note: I don't want alternative designs, or input from people who just don't know. I'm understanding theory, and I need accuracy and certainty.