For xblocks not being there on rebuild
If you want that your XBlocks are there when you re-build you can check out this post Install xblock Open edX tutor Quince - #2 by chintan
Difference between openedx and openedx-dev
Yes those are different images, in dev mode it’s easier to do development and it restarts when mounted directories are changed. It also allows you to put in pdb. If you want to do development of your XBlock openedx-dev is the image to go for.
k8s/dev/local difference in setting up the XBlocks
You can just put the blocks the way it’s mentioned in the link and tutor will pick it up for all of them. However for k8s, I am not sure if local Xblocks (within your machine) will work or not, it might need to be on git so it can pull.
When I am doing the XBlock development, I prefer to go with tutor mounts add... command, and start it in dev mode, it’s easier in that manner.