Definitely use the latest official version, 2.1.3. Maybe you have to upgrade your Linux Mint to have it in the repository. Or even better, get the development version 2.2 rc, it's stable. But I think you will have to compile it yourself, can't help you with that.
You can select the window only by just pulling up a window around it. Yes, I strongly recommend to use a mouse for CAD.
Alternatively use "Select" - "Select Window".
You might also draw the windows on an extra layer and lock the wall layer.
Use "Modify" - "Fillet" to round corners, one by one. Set the radius at the Tools Options Bar above the drawing area. It's contents vary according to the active command.
To copy the window use "Modify" - "Move / Copy".
Experiment with the snap settings, always have "Free snap" on, often better "Snap on Entity" off, as with it on "Snap on Middle" won't work.
When drawing a rectangle with the "Rectangle" command, it becomes a single entity, and some editing commands won't work on it. You will have to "Explode" it then.
These source code files can't simply be installed. You have to compile them yourself. That's something for experienced linuxers. I can't do it. If you have the knowledge, or a Linux crack available to assist or do it, you can try. Otherwise better first install the latest Mint and look what it has in it's repo. For Linux 2.1.2 is O.K.
Or search the net for a .deb file. You will need two files, the main librecad-xx.deb, and the librecad-xx-data.deb. And a bunch of dependencies, mainly qt5 files, if not already on your system.
If you have Wine installed, you might use the Windows version. Easiest the portable 2.1.3 from portableapps.com. Works well.
If you are able to get 2.2.0 Release candidate installed, then skip 2.1.3.
The .dmg file is for Mac, I think.