Right. There are many Oracle internals experts who are presenting at conferences, they are also quite approachable. I'd especially recommend Tanel Poder, several of his troubleshooting sessions are available on youtube.
There are several unit testing frameworks for PL/SQL. Code in the database encapsulates business logic, it gives you unified access from all clients. It can also be orders of magnitude faster. The presentation linked here
That's not how it's supposed to work. You want DBAs to be able to tune the existing data model with indexes and materialized views. The optimizer should be smart enough to rewrite queries and, in your case, look up the necessary data from the index only. I don't know Postgresql, Oracle's optimizer has been doing that for years.
This reminds me of Nassim Taleb's barbell risk management strategy: Avoid the middle in favor of the extremes. For investments, it means a combination of very conservative plus some high risk investments to win big. He applies it to many other areas, though, including health and politics.
Disclaimer: APEX dev team member here. Independent of APEX and even the Oracle database, if you are not using stored procedures, you are not using the database efficiently. See e.g. this presentation for a performance analysis: https://www.youtube.com/watch?v=jfkph4lXmKU
Right, and just in case anyone is in doubt, the ratio for cattle is also 50/50. There is no sexing that I am aware of, but if someone is a vegetarian and drinks milk, they are still supporting the schnitzel industry.