1, Microsoft should add an API to allow apps to declare content to be private/non-indexable. At the same time, they should add overrides in Recall so users can choose to force index or force ignore specific apps regardless of how they mark their content. The power should be in the hands of the user. If the user indicates they want to index their Signal activity, it should be done in an unblockable way. Conversely, the default would allow for ignorant users' privacy to be respected. And Signal would not have to engage in unsupported behavior that could potentially break down the line (imagine a future Windows where rendering to Recall and the screen is treated the same to simplify the code base and reduce bugs).
2. When I run software on my PC I am granting it permission to use my property to perform some task. If that program goes off and starts doing other things I may decide it is violating that trust and terminate it. That is my right as a PC owner. Signal is interfering with Recall's operation which doesn't really fit in with its basic premise as a messaging app and if it is not configurable this can be seen as sketchy behavior. Since they have announced this behavior publically it indicates it is not meant to be hidden behavior which does make the behavior more legitimate.
3. Apps should not claim that content that users can see cannot be permanently stored. This is at best naive and at worst a bold-faced lie. For example I could take a picture of my computer screen and Signal would never know. And of course the person being spoken too can remember what has been said. Users who are under the impression that what they say will have no consequences may end up behaving differently than if they were aware of the reality that anything they put "out there" may very well never go away.