I have an opening in a wall which is from the floor but not all the way to the ceiling. Is there a way to model openings in the wall or do I have to simply not put a wall there and then create some furniture which starts at an elevation?

Use the models named Door frame or Service hatch in the Doors and Windows category with an elevation equal to zero.
If the top of the opening is round, use a Service hatch and an elevated arch as suggested in this example.

Is it possible to group objects and save them as a group (as opposed to multiple select?) I saw that grouping is something which was added in version 2 but I am not sure if it is limited to multiple select.

Grouping will be added in the coming versions. At this time, the only way to group objects is to copy/paste them in a new empty home, export that home to OBJ format with 3D view > Export to OBJ format... menu item, and import the exported file in the first place with Furniture > Import Furniture... menu item.

while on the subject of group, is there a way to rotate and scale a group (either in selection or as a saved group if available)?

Same answer and same tip...

Somehow I managed to create copies of an entire area. Could someone point me as to what are the keys for that (so I'll be more careful) and if there is a way to find out where it happened (other than start moving walls and see if another is beneath them)?

You can duplicate any object by keeping the Ctrl key pressed during a drag and drop operation (or Cmd key under Mac OS X).

I need to make a round wall. I saw in the FAQ that currently it is only possible as a large number of small walls. I was wondering what effect if any would be to simply create a furniture instead of the wall and if there is such a furniture ready where I can download and use it or do I have to create one myself (and if so any suggestion on how to do it without having to learn too much...)

You idea about a 3D model looking like a round wall part is probably the best idea, and I should have thought about this workaround a long time ago. [;)]
I just added a new model in 3D models import page, and will update the FAQ accordingly.

Is there a way to set the default texture of walls and furniture (instead of white) or do I have to add them each time?

No, but you may select all the items your want to modify in plan, and choose Furniture > Modify..., Plan > Modify walls... or Plan > Modify rooms... out of the menu to modify all the selected pieces, walls or rooms.

I have many walls in an angle. I saw that it is possible to see and change the angle when creating the wall, is there a way to do it when the wall is already created (other than calculating the angle manually and then setting the position accordingly)?

No. It's not that it's too complicated to program, but I feel it wouldn't be that useful.

Also with walls in an angle, sometimes windows and doors do not snap to a wall in an angle (even when the magnetism is enabled). Is there a way to manually do it?

I guess you tried to change the location of the end points of a wall, and then the windows or doors placed on this wall looked incorrect, am I right?
If this a case, you'll have to move again each window or door on the wall so they are correctly placed into the wall.
If I didn't understand you, please give more details.

When I do measurements in my house I measure the inside of the walls but when I create a wall in sweet home I enter the full length (which is from the middle of the adjacent walls). Now if I am on the grid it is easy to calculate, is there an easy way to do it for angled walls also?

First note that the easiest way to enter the exact length of a wall at creation time is to press Enter key, and then enter values with the keyboard. If you want to draw a room with a special shape but can't import its plan as a background image, I would create it with the Create rooms tool, enter its measurements with the keyboard, select the Create walls tool then double click in the room to create walls around it.

Is there a way to manually connect/disconnect walls? Currently I can only connect walls when I initially create them and I don't know how to disconnect them later if I need to.

To disconnect a wall use Cut / Paste. There's no way to connect an existing wall at this time.
Read also this thread.

Does someone know if there is a model of a sliding door somewhere?

See in Bedroom category. These sliding doors are not among Doors and windows category because they don't create holes in walls.