Re: downstairs toilet bubbles when upstairs is flushed
Sounds as though this a recent occurrence
Sight unseen I'm guessing the upstairs bathroom is more or less above the first floor bathroom.
If so .... they're likely sharing the main soil stack.
By the sounds of the issue with the main floor toilet not flushing properly sounds more to be an issue with a partial blockage somewhere in the drain pipe close to where this toilet joins the main stack.
The bubbling of this toilet can also be caused by a blockage.
Anytime water from the shower , sink or toilet is flowing down the drain line .... a volume of air is being pushing ahead of it.
When the second floor toilet flushes it's moving a large bubble of air ahead of the discharge down the soil stack. When this hits an obstruction the bubble of air will flow back. If your main floor toilet is the closet fixture to the stack the air will flow back up into the toilet.
If you have a clean out access to the stack in the main floor bathroom .... remove the cap and with a snake try and dislodge the obstruction.
Otherwise if no clean out access you may need to remove the main floor toilet and try to snake the drain.
Just a thought.
"" an ounce of perception -- a pound of obscure "