JEngledow – i can send you one to borrow if you want as long as you send it back.
We had same issue, we flew back from NZ and my daughter (20 mths) had a seat all the way back. The domestic leg was a nightmare as they refused to believe we had bought a seat. It was _exactly_ what we needed after 32 hours …. Surely you can buy a seat for whatever you want, like your prize viola.
EDIT: the excellent part, and this was Virgin, was them allocating her a seat in a totally different part of the plane to us then refusing to help move her. So we took her on plonked her down, belted her up and walked away. The look on the womans face sat next to her was priceless. She didn’t care, she was happy sitting there on her own. for 9 hours. In premium economy. Honestly, it was great. Her mum and I got really plastered while she screamed all the way home*
* of course not.