Hi Aniko,
I'm sorry to hear about your difficulties.
There are a number of things going on here, and I'll try and answer some of your questions, and perhaps revisit other parts at a later stage.
I'd like to address the following:
1) Body dysmorphia
2) "weight gain"
3) Laxative abuse
First of all, I'm amazed that during your inpatient stay, laxatives were even permitted! I tried to smuggle these in, and the nurses knew literally every trick in the book...low and behold these were confiscated.
It is also important to highlight that developing eating disorders later in life is perfectly common especially in the early stages of your adult life as there can be lots of challenges you face outside the safety net of your family home. I actually relapsed significantly when my parents moved from our family home and I was lost in the world of renting, my first job and establishing a social network in London. So be kind to yourself as it is certainly not just teenage girls who suffer with this (and then magically goes away once you turn 18!)
Body dysmorphia.
You have highlighted that you feel bloated, that your thighs and tummy feel enormous.
There is a behavioural reason for this. Our bodies have been conditioned to react to danger e.g. bear in the woods (or more hilariously, lioness when camping in a serengetti campsite...yes these things do exist...) I woke up to go to the loo in the middle of the night, and saw what I thought was a lionness and jumped back into the tent screaming. I had feared the lioness was a few metres away from me, it was huge and it was going to attack me. And more importantly I saw it as a lionness. Actually, it was a guard dog and was a lot further away than I thought but my brain had told me it was danger and therefore I needed to fight or flight - in my case flight. In the case of body dysmorphia, the exact same thing happens, and the lionness is replaced with being large (I don't like to use the word fat, because feeling or being fat is not correct. Fat is a naturally occuring tissue in the body and whilst some people have more of it than others, you cannot feel fat). Therefore your brain perceives what scares you to be bigger than it actually is, and closer. This is hampered even more when you put your body at risk such as low weight, restriction etc. Your body goes into fight mode because it senses danger and therefore is in a heightened state of anxiety. This is why your organs also go into "safe mode". Your metabolism slows down, you can experience breathing difficulties, you feel cold, your concentration goes etc etc. Your body is trying to conserve as much energy as possible. The heightened anxiety means that at a low weight you trip yourself even more. Ironically as you get healthier the anxiety does decrease, although it is natural to still feel dissatisfaction with the body.
With your metabolism (and I am taking the laxatives out for the time being), it will have slowed down substantially and therefore as you gain weight, it will be confused and trying to adjust, so you will find weight gain occurs in odd places, and that you will start to need even more food because your body is trying to rebuild. This is why in inpatient units malnourished patients will eventually increase to around 3000 calories a day, because you need more than your recommended daily allowance in order to gain weight.
2) Weight gain
Weight gain is a funny one. As I just outlined above, you need to be eating more than your RDA in order to gain weight. Now this will also happen as you start eating more (and below your RDA), because as I mentioned your metabolism needs to recover, but also tries to conserve energy. So yes, more food = body mass gain.
Your weight is a transient thing, moving up and down all the time when you eat, drink, sweat, go to the loo etc. It will not stay at the exact same number. Women experience even greater fluctuations during during their period (mine can increase any where between 1kg to 3kg, because I retain a LOT of fluid, but I haven't gained weight, I've retained water, and it goes back down again after). There is some mass gain in that obviously, because my meal plan is trying to increase my mass, but if I were maintaining, it would just be changes in fluid.
so this is important to be aware of, but even more so with laxative abuse. Laxatives don't cause body mass loss, although people think they do because your weight goes down on the scales after you have taken them. You mentioned that as soon as you eat or drink, your weight goes back up again. The reason for this is all your have lost is fluid. Like when a person first starts a crash diet, the first few days will see an enormous decreasing in their weight, because the body loses fluid, then it can be frustrating because the weight loss is not as great as before. Furthermore, when the person eats properly again, their weight bounces back up because it has regained lost fluid. This is all that is happening with laxative abuse and will explain your loss and gain. You will also feel way more bloated because as the laxatives act on your stomach, it pushes lots of water down into the intestines before emptying more than is normal.
3) Laxative abuse
I probably don't need to tell you how dangerous laxatives are, but in addition to the water loss you lose core electrolytes which are essential for the heart to function. Laxatives can be fatal for this very reason. You can also do permanent damage to your liver and kidneys and many long term abusers need dialysis as a result.
When I was admitted as an inpatient, my consultant was very clear that another week and I may have died. I have also more recently in 2014 been admitted because of my critically low potassium levels...a consequence of the laxative abuse.
It infuriates me that laxatives are readily available and not behind the counter. If you buy a certain amount of paracetamol then you will be stopped, because paracetamol overdose can be fatal. Laxatives should, but don't, fall under the same regulation.
I would decrease your laxative use bit by bit. You will be constipated, because your body has relied on laxatives to empty your bowels, and not natural processes. Reducing bit by bit will help get your bowels working again, but you will need to be patient with it. I would also speak with your doctor to work a plan on how to reduce these safely.
On recovery. Sometimes, we aren't ready to recover. When I was inpatient age 18, I was not ready. I had to, but mentally I wasn't, despite the unit being brilliant, and my consultant one of the leading people in the country. I am still under the same consultant (and funnily enough the terrifying dietitian I had in hospital, who is SO much nicer than I remember). But the difference is I am determined and committed to recovery. It doesn't mean it is any easier, and at times extremely tough going, but it is like a full time job, you have to work and work at it. But, it is worth it...I am getting there and my life is so much better than it has been. I played my cello yesterday for the first time in months (as I've been studying part time for an MA alongside recovery and working full time) and whilst I got tired fairly easily, what amazed me more was how on earth I played so well and such a high level when I was so poorly. I am fairly sure it was adrenaline that got me through, but I remember constantly being in a daze or trance, and there are huge parts of my adolesence that I can't remember or am in disbelief about how I survived. Recovery is so worth it, because you can live.
Hope this helps, do message me if you have any more questions or concerns.