I assume one of the problems here is that you don't use an USB-C to USB-C cable in combination with using the "USB controlled by this device" mode.
"USB controlled by this device" should be equivalent to what was called OTG with USB2/Micro USB. At that time you could not use OTG with charging at the same time.
In the USB-C era it should be possible if the ...
Sorry, you can't do it the way you intend to.
Google photos can back up to Ext.SD, only if the card is treated as internal storage. In your case the SD card is not treated as a part of device memory (portable) .
Adoptable-storage is applicable only if the card is actually mounted on the device and not when connected via USB connector.
Yes an no.
Some years ago this was a very severe security problem, as a lot of devices automatically enabled the MTP mode when connected via USB that allows to access all files in the "sd-card" section. Also some device had ADB enabled by default.
Nowadays MTP and the other file-access protocols provided by Android devices are usually disabled by default ...