Hello @onat, many good questions, which all have been discussed here on Connect many times before. I am sure, you will get some good comments, but no real solution except for what you said:
This is actually, what I do since three years and the good news is, you and I are aware of the fact that some of our contributions are rejected (many other local guides do not know that at all) and we just need to keep those under control.
My understanding, why Google doesn’t tell us any details for what reason exactly a contribution was rejected, is because the filter not only check for obeying the rules, but more importantly for suspected SPAM. Giving out more detailed information, would help professional spammers to bypass the checks.
Anyway, if not already done so, I suggest reading How to Get In and Out of Trouble: The Main post.
I usually check My Contributions right after uploading which takes less than a minute most of the times. Then I share the link in an anonymous session, to see, if it became public. For photos taken inside the European Union (EU), it is easier, because hidden photos are marked in My Contributions.
However, recently, we found a problem with photos made with certain models of Galaxy smartphones. In that case, it is enough to delete the uploaded, slightly edit and upload again. Have a look, if this applies to you too: Cannot upload images to reviews - #6 by WilfriedB
Hope this helps.