If she seems to not physically be able to do it, then I would look for an SLP (one who also does feeding issues) rather than an OT. This sounds very much like my DS, who had some oral motor issues and hypotonia in the mouth. The EI folks didn't even notice these issues and so they did no work inside his mouth - with no results. I suspected there was something wrong with his mouth (for eating) as early as 8 months. He pretty much gagged and threw up anything he would even try. Then he stopped trying. I started looking for help at 10 months, and EI took 2 months to even get here. In hindsight if I had it to do again, I would go directly to an outside therapist. The EI program in your area may be better and faster than the one here. Some ladies on this board have had success with their EI program. You can also try that in parallel with any other therapy you get privately. It's worth trying anyway.
BTW, my DS also had trouble BF-ing from birth. He did lots of chewing on me as a defensive thing against something being in his mouth.
In any case, listen to your momma instincts. If you think she can't physically eat, then try to find a reason for it, even if you get some "nothing wrong" answers at first.
If she is getting frustrated trying the purees, I would not force them. In fact, forcing any food has always led to backward progress for us. But - since she seems to be wanting to eat, try to get a therapist soon so you can work using that desire to eat, but only when she can physically do it. No sense trying until any physical issues are being worked on. Otherwise she'll just get frustrated and quit wanting to eat.
Keep doing what is working and supplement if you have to any way you can, until you get help. Can she take a bottle? If not, can you try working toward a cup? That way you can pump or give formula for supplementation if she's not BFing well. Perhaps LLL can help a little here.
Get yourself strengthened any way you can - it's a long hard road ahead of you. The good news is the sooner you can address this properly, the easier it will be in the long run for her to eat. If this is your first child, don't let anyone tell you that "you're doing it wrong" or "your BFing is causing her problems", or even worse "she'll eat when she gets hungry". I had plenty of that kind of attitude from others, particularly family, and it's all wrong. You know what's best for her.
I just read some of your other posts. The sippy is good for giving pumped milk. Before he was a year old, I only had a manual pump and was doing a lot of pumping. I felt weird buying a real pump when he was already a year old, but you know what, it was the best thing. I was able to add a tiny tiny bit of either baby rice or baby oatmeal cereal to the milk and feed it to him by spoon, as he wouldn't take pumped milk in a bottle or sippy. Then, I started thickening up the oatmeal -excruciatingly slowly, over many months-. I think that helped him ease into texture a bit. However, I stopped and eased up on texture if he ever seemed like he was having trouble.
One thing that helped my DS gain weight was getting him on full fat, smooth textured yogurt. I started him on it at 18 months, though I probably should have done it sooner. Just be sure it's very very smooth. I gave yogurt to DS twice a day, morning and night. It actually didn't slow down his BFing at all to get all that yogurt. He was hungry, he just couldn't eat many things. At 18 months, he was BFing a lot, eating the yogurt, and eating the tiny bit of cereal in my pumped milk. Occasionally he would eat watered down and very smooth sweet potato puree, just for variety. That was it. Anything else and it was gag-city.
You may find that you have to give up lot of preconceived notions about feeding. At some point you will just want her to eat anything, rather than some ideal set of food you think would be great for her.