Hello, I think all your issues can be addressed.
1.) There are several Ubuntu varieties available for eMMC install. They can be found on Khadas Docs firmware page. You will use an image name starting with VIM in the name, do not use image names that start with VIM2.
Server and desktop versions are available.
Once you determine which Ubuntu version you want to use, download the file. File will be a 7z(zip) file, extract the file. This will result in an image.
2.) The image can be flashed to the eMMC with one of two methods,
1.) using the Amlogic USB Burning Tool method
2.) using the Burn Card method.
If you need assistance after reviewing the guides, we will try to help.
3.) Buttons will be used to enter flash mode, this is explained in the links to Burn methods above.
When running Ubuntu from eMMC, no buttons(other than power button) will be needed.
Note: User @balbes150 has Linux varieties that can also be installed to eMMC. He has a script that will backup the entire existing eMMC to SD card. Info on this can be found here. Basically, it involves booting Ubuntu from SD card and running the ddbr script. The backup will be dumped to SD card. If you need further assistance, we will try to help.