I'm sorry I forgot to say that will be a wonderful chair! So bright and pretty!
That would be worth trying, but only on a small sample board. If it worked, then she could use it on the chair. I once decoupaged a small bedside table. I put marine varnish over it, and the entire piece crazed. We don't really know why but we guess it was a reaction to the glaze I'd used on the decoupage, or the glues coming through. I left it that way, and over time we came to like the old antique look!