Hi Albaba sorry you've felt like a thread-killer, I have been away with work all week. We are all still here but we do dip in and out
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Our dd2 is 2.5 years old and she started sitting at 18 months and at about 2 years was pushing her bum into the air as you describe. Then she went on what I consider to be the typical walking development path, only much delayed and at a slower rate of progression. If she had been left to her own devices, I am certain that she'd still be rolling to get wherever she wanted.
DD2 could weight bear fairly early on, so we'd position her on her feet (too difficult for her to get up to standing position) and put toys on the sofa so she was entertained while standing and she leaned forward on the sofa for support. Then we'd put choc buttons/TV remote control/any desirable object on the other end of the sofa to encourage her to sidestep for it while holding onto the sofa. We started the sidestepping by moving her feet for her with our hands until she got the idea. Then holding both her hands and taking a forward step. She's now able to take about 40 independent steps, can climb out of her (very low) junior bed/cotbed without bars, and using the wall as support, stand up and open her bedroom door.
Her new trick this week is to climb up stairs. It takes quite a while to get her little foot up and posited on the step before pushing up; we've/I've done a lot of work with her to position her foot and then tell her to push up as I supported/gently lifted her torso so she stood up. We've been doing this for about a month, now she can (very slowly - one step, then rest, one step, then rest) climb. She now positions her foot by herself, and then pushes up with her little legs, while her hands are holding onto the stairs, 2 steps up. She pushes our hands away if we try to help now

As Muchado says, we also do lots of physical non-physio work. Dancing (bobbing up and down bending the knees) holding hands to all the CBEEBIES songs (music is a great motivator). Ring a ring of roses (side-stepping slowly round in a circle holding hands, then down to knees and then I lift her back up into the air that she loves. We do Portage swimming which might be considered hydrotherapy (?) where she's encouraged to kick her legs, splash with her hands, float on back and front, 'jumping' in etc. I'm enrolling her for 'Water Babies' in the new year to get more time in the pool for us. She likes her toddler trampoline (holds onto bar and bounces from the knees) and her trike (feet are resting on peddles and grown-up pushes trike to make her feet go round on the pedals).
I think your daughter is showing positive signs of walking and you're doing all the right things by encouraging her onto her feet. I hope my experience above has given you a little hope, but it hasn't happened overnight. We've also found the development happening in spurts.