Will the back of the first dining room chair look unfinished?
Just from viewing the video myself and doing some improvements like this, my answer to your question would be “YES”. You could probably solve the problem by measuring the back of the chair across and then length wise and them purchasing extra material. Use the extra material and staple it over the entire back of the chair board around the edges of the sides and top and this would cover any areas which may show through. At least that’s what I would do or I guess you could fold under the material stapled on the back and add some sort of decorative design like Kelly used on the second chair. Good luck … maybe you will get better answers later.