Some thoughtful suggestions there.
[ I do not represent DW. Am just a user. ]
A couple comments on Scripting and Object Transistions...
...after some playing around with Scripting I found that I can Drag&Drop. Over on the left of workspace, click on one of the lower vertical tabs that lists script functions. When I am editing Actions for an object and building a small script, I just place my edit-cursor and then drag the Script function (from the far left) to the action/script space. Did not try it for script objects.
...when I build up an Object with transitions and other properties, I add it to the Gallery. I find that I can later Copy those actions to certain other like objects, or place the Gallery object into the current page. (Probably not what you meant by custom transitions?)
**[edit] Correction for the above. When writing Script, Double-Click on the Script term in the Organizer (left Panel) to insert it. (it's not a drag & drop placement). You still need to type or paste the object name into script line, and maintain syntax. Of course, this applies to Opus versions that support Scripting.
I'm wrestling with DB myself, but wouldn't even know 'what' to ask for.