Why do you think the current behaviour is incorrect? If i understood it properly - it is perfectly aligned with the visibility rules - if you as userA run the query "select id from test_table" - you'll either get error, because the table is in another schema? For that reason, you can't right...
Regarding your second question - i think the most similar approach would be to put the query 2 (or more if required) times into sql window, run it and then select 2 result set tabs (by Ctrl+LeftClick on the tab) - it brings both tabs in foreground, both result sets can be fetched then separately...
Editing the procedure in broader sense might include changes to api (renaming of parameter, complete new parameter and so on). To reflect such changes - the test window should completely be reloaded discarding non present parameters and adding new parameters. As of know - i don't think, such...
I just published a prototype for unwrapper addon based on excellent luaplsql plugin by Nodir Temirkhodjaev -https://github.com/Maxim4711/luaplsql_addons
There is a short README with description how to build it locally, alternatively - i provided a zip release for 64 bit version. I somebody...