Margaux, That "scrum" app is just one possibility, and a minimalistic version. I think the answer to #2 is no if you are talking about the tactical, in whatever format, yet there is an answer that helps you with this.
#1 - I say yes. My interpretation is that you don't even need a policy to modify tactical if you keep the constitutional process...but the last sentence here leads me to believe you could really change it completely via governance:
As for secretary and facilitator, you could constrain the election of facilitator to those filling another role, or say that whoever elected facilitator is also fills another defined role to build in the facilitation piece. You could propose adding accountabilities on Secretary for scheduling the modified meeting.
If the meeting replaces tactical then I don't think you could remove secretary authority to schedule, but if it does not explicitly replace it, I could see another role with an accountability for scheduling it.
I would be mindful to not infringe on the constitutional authorities granted to the elected roles. Curious what else people have done here! Let me know if you have further thoughts.