Longitudinal frame twisting and different kind of holes in transverse frames

I understand what you are saying now.

 

My rule of thumb for mouse holes is reality based having cut them in steel plates, I beam and angle bar ends building ship structure. We have no idea of the scale of
the OP's hypothetical design to say how big of a mouse hole.

 

As already noted, the mouse at the base, is to allow the welder to perform a return around the web. This prevents cracking, if left as a stop/start at the end of the mouse hole.

As an extreme example, not performing returns, can lead to cracking in other locations too, like here:


In general mouse holes/cutouts are not to be greater than 50% of the web depth, it is passing through.

As for shapes and guidelines, there are many, such as:
DNV's


LRs


And you also need to be careful when creating the shape of the mouse hole and its objective, since this relates to cracking from fatigue.
Such as:


It is a very very deep rabbit hole, if you wish to go down this route.
Viz:-

 

Thank you very much for this more detailed answer!