I did pretty much the same, jumping directly into high Huel % days from a diet of mainly sugary snacks, and the occasional cooked meal.
So yes, the first question, yes it realistic to jump into Huel 100% right away, but there might be some side effects until your body adjusts. The same could be said going from a macdonalds diet to a mediteranean diet, noone will argue it’s worse for you, but there will be some side effects!
If you are taking in the same number of calories, as you expend, then no you will not lose body fat just like that. You could mix with exercise to tone up, and convert fat into muscle ( muscle burns more calories than fat) and reduce your size while staying the same weight, lowering body fat % in the process.
Feeling hunger is a weird one. I can sometimes feel full up, yet empty. It is knowing the difference that is the key. I’ve yet to feel properly hungry on Huel.
It took me around 2-3 days for the sugar to clear, and then a further 3-4 days for some other side effects (Mainly Gastro) to resolve. Now I have no side effects, but I’ve got loads of energy, motivation, and feel so much better than I did before.
My only thing to note with your 1000 calorie suggestion is that from your info, your resting calorie burn is around 2100 calories a day. a 1100 calorie deficit will make you feel very hungry, and might be hard to stick with. Especially going from a “not great” diet, you will be used to temptations, and with feeling so hungry, you’re more likely to give in.
1600 calories a day will make you lose 1lb a week, which will make you feel more full, and give you more nutrients and minerals. This will equate to around 400g of Huel a day, easily divided into 4 meals of 100g. Or a bit more ease of use, 3 meals, each of 3 slightly heaped scoops.
Hope this helps!