Is there any point of getting Tifas Underwear in her house when clouds..

No, theres no point in having them, you don't have to use them in any way.
As for sephiroth, your right thats how he was defeated by cloud, but you could say he never really died though.

No, there's no point in getting them. You don't get to play the piano if you do, which stops you from getting Tifa's Final Heaven limit break in the future unless you already know the code.

Sephiroth dies when you kill him at the very end of the game.

There is no point really in taking tifas othapedic underwear out of the cabanet except it might be funny and that it will also contribute to who you go on the date with at the gold saucer. The best person to go on the date with is Aeris because you get to do a show and it's really funny if you say the wrong things.

Sephoroth dies in the very end. You will defeet him a first time then he will go into a second form. Then you defeet him again. It looks like he dies but he didn't. You will actually have a battle again. This time it is not you playing cloud it's in one of those movies. He will use his limit break and sepheroth will die. If you whatch the final movie you will see that every one gets out of that big hole in the ground except for sephoroth. Also i've beeten the game aboute ten times and i've never seemed to be able to beet his second form unless I had omnislash (clouds final limit break).