The piece in the drain is screwed into the drain under the tub It is usually corroded and is not going to unscrew. You can use a sawsall or jig saw to cut the drain in 3 or 4 places around the opening and peel the piece out. You can also disconnect the drain under the tub and just leave it on the tub. There should be access to the back side of the tub. Usually a panel or door in the back of a closet that is in the room beside the bath. If there is no access, you will need to make one so that you can connect the drain to the new tub.
The tub and toilet connect to the same drain at some point. The main drain was plugged but the connection between the toilet and tub was still open. Water seeks it's own level. When the water came up in the toilet, the tub was lower so some of it went there.
Once the old tub gets into the drain it is impossible to remove.........
Usually. Going through the overflow drain gets you straight into the drain without having to make the sharp bend in the tub drain. There could be a trap that comes apart in the drain underneath if it is accessible.
Connection of the washer drain hose to the tub drain is very important. This will help prevent leaks for example.
You can expect to pay about 25% more for a walk in tub. The walk in tub is larger and holds more.
15-16 inches from back wall. 8-10 inches from side wall tub drain is on. Measurements are to center of tub drain hole. Measurements are close, not exact, and are for a standard tub.
Use a snake and unclog the drain PROPERLY. Do it quick before everything is eaten away on the tub and drain.
One the end of the tub above the drain there is a plate. It may be where you close the drain off or it may just be a cover plate. It is open on the bottom edge and goes into the drain so that the tub cannot overflow.
You would have to build a platform for the tub to sit on. About 3 inches if you are using a floor drain. There is a trap right after the floor drain so you would not need one at the tub. If possible you could position the tub directly over the floor drain but that would eliminate the floor drain which would be a problem if the basement every flooded.
Yes as long as the drain has a trap and is vented
There should be a drain valve where you can hook up a hose and then open the valve to drain.