Hi,
Welcome to zoo of parameters! (lol)
Your quest seems mostly focused on RSSI logic. I don't have all your answers, but
I like to point out that there are some other important ways to affect parent selection:
- mesh topology
- maximum layers
- max_connection per node
- fixed root
Some of these ...

My setup:
My code is derived from examples/mesh/internal_communication.
It is WIFI -MESH without router, I set fixed rootnode, all messages sent are with P2P.
The mesh connects OK, 3 nodes in chain, close together as test.

The scenario:
At some point, a node sends a message to the rootnode but its ...

According to the API documentation esp_mesh_send() is not reentrant. You can use it in multiple tasks, with e.g. mutex, but then it will still be sequential. So, I would say the answer is 'no'.

I have been trying a similar thing on the ESP32-S3-WROOM-1U P1N8R8, using IDF 4.4.2 and I confirm the issue.
My conclusion is:
- the documentation seems incorrect : the PSRAM is not mapped at 0x3D000000
- it is mapped at 0x3E000000 - size(PSRAM) = 0x3D800000 in my case, works fine like that :)
- the ...