I think the South facing window at 70f is your problem. They need to be as cool as possible, in bright light but not direct sun. The flowers are ephemeral anyway but you can keep them going as long as possible by following the cool and light mantra. It is always best to buy potted bulbs in closed bud so you get the longest possible bloom period. If the flowers are already open you'll not get more than a week or so out of them even under optimum conditions. There is no point potting them into a bigger pot. It would disturb their roots and shorten their life rather than extending it. It would be like taking ice cream out of its tub and hoping it would last longer in a big bowl. Potted tulips indoors are essentially disposable, although you could try to keep the bulbs to flower again next year.

Potted tulips......all tulips.....bloom for a very short period of time. They are intended for temporary decoration only. Not your fault that the plant is in decline. Toss.

I agree with getting them before they actually bloom (or at least, just starting to bloom). But disagree with tossing them. After the blooms have died, you can transplant it and let it keep growing.

It's better to wait until all the leaves have also died back before transplanting, but that is in the case that you feel rushed to get them out to another pot, then at least wait for the blooms to go.

You can also transplant them into some spot in the ground and hope they come back next year for you. Tulips in the ground may do this for a few years but to really get more tulips, you may have to replenish them every few years.