It's just about possible for me to feed her on Rob's lap. Neither of us can do it alone.

I do sympathise. I could never get food on the spoon properly without holding the bowl and you can't hold the bowl without YB wanting to guddle his fingers in it.

If you're not going to be leaving L for long periods in the chair, especially unattended, I wouldn't worry. YB started solids at 6 months, in his highchair, and couldn't sit unaided. We used to have to hoick him upright at least once per meal because I couldn't get cushions in the chair with him.