I know it is the hardest thing in the world to hear a child cry in the night, but it sounds to me like she has discovered exactly what to do to get what she wants. She wants you there in her room as long as she is awake. Understandable, life is always more comfortable with company. However, you clearly do not want this, it is not good for your health (or you back, lying on the floor). It affects your sleep, which in turn affects the entire next day. You also know that she needs to learn to comfort herself and learn that mom and dad are not at her constant beck and call. She needs to learn that you will always give her what she needs, but not always what she wants. At 2 1/2, her crying/screaming is not an attempt at communication, as with an infant, but a clear manipulation of her environment to get what she wants.
This is what I would do. I would pick a time when you and your husband don't have any pressing needs during the days, like a weekend (3-day if you have one soon). Send your son to Grandma's or somewhere for the night. Be sure you sit her down and explain to her there is a new rule in your house, and that is that mommy or daddy will check on her if she wakes up, but will not be sleeping with her on the floor or staying with her. Explain how much you need to sleep in your bed in order to be healthy and happy, and she needs to learn to go back to sleep on her own. Explain that the new rule begins tonight, and there will be no exceptions to this rule. You may want to get her a special new sleep buddy or something. (My kids have a "dream washer" that they can tell bad dreams to right in their beds and not even have to come to see mom and dad, although they sometimes still do.) Then, put her to bed and brace yourselves. When she wakes up in the night, check on her once, with a kiss and hug and reassurances. Then leave. Let her cry and scream. If you know she is in a safe room, and her diaper is not poopy, and she has had food and drink before she went to bed, than just let her cry it out. If she is still screaming, you can certainly check on her every once in a while, but do so without engaging with her. Don't talk to her or reward the screaming in any way. Just make sure she is not injured, etc., and then leave again. Do not give in. Arrange things so that if this goes on all night, you can deal with it. Because if you attempt this and then give in at any point in the night, she has won and you may never get a good nights rest for a long time. At her age, she may well keep this up the whole night. The next night, same routine. Remind her of the new rule before bed, and when she wakes, hug and comfort for a moment, then leave. Do not reward the crying.
Don't get me wrong, if she has a bad night, and wakes from a nightmare several times, you can comfort her several times with hugs, kisses, etc. But only do this if she has been sleeping and wakes up for some reason. But what you do not do is reward the screaming/crying with hugs, kisses and company. It may take several nights for this to really sink into her mind that the rule is permanent and you will stand by your word. I believe that 2 or 3 nights should be enough to alter her habits, and assure you of better sleep in the years to come. Just remember that after the habit has been changed, there will probably be several times when she tests the limits again, seemingly at random. Be consistent, even 3 or 6 months later. Good luck.