I hope the tube works for you. I'm incredibly underweight. My BMI was 10 at the start of the summer, and I've gained a few kilos since then. If it's any reassurance at all, I just underwent major, emergency surgery and am fighting off an infection, all at this weight, and my heart's doing fine, touch wood, and so are my other organs, so have some hope in the body's capacities for resilience.
My experience with the NG feed was bad, but nowhere near the TPN disaster bad. If the worst happens, NG tubes are lower risk and easy to take out if needs be. My stomach just didn't like the feed for some reason, but I think that's very individual.
Now I'm back to gaining weight with just good old fashioned food and very happy to be doing so. Eating is easier since this surgery - I had problems building unbeknownst to me inside, and although I still have gastroparesis, I'm tentatively optimistic about gaining some weight now. I think there often are solutions out there if you can just find the right one(s) for you - I hope the tube will be that for you! I get weighed tomorrow morning and am anxious to know what it's going to say!