Making a generic drawing function is useful if you intend to use it in more than one place (like gizmos for example). Otherwise I don't see the problem is to leave the code in the transform operator.
(That's more personal taste)

This is an area I don't remember ever touching before.
I realize that the code can be deduplicated with ELEM(t->spacetype, ...).
Some transform modes use these handles more than others (like Rotate for example).

A little weird to edit a tsnap member out of place.
Maybe it's better to make a dedicated drawing function for this mode, as we have the: drawEdgeSlide, drawVertSlide and drawDial3d

Changes in the state of scene properties are best located in saveTransform as this function only runs on success and an undo step is appropriately added.