I think it will look fine if you paint the risers and the spindles. But don't just paint one...you need for the color to be repeated somewhere in the staircase to tie it together.

A 4 1/2" profile on crown would give a drop of 3 1/4" - 3 1/2" which, IMO, would look skimpy with 9' ceilings. What your carpenter did by adding a backer board with a profile is frequently done in our area (and appears to be used frequently elsewhere given the number of photos on Houzz of this treatment). If the size appears to be proportionate when looking at it in person, then the product looks fine once it is caulked and painted. If the total affect still looks small to you, I would stop now, return what you can and go with a larger crown. You can do a search here on Houzz for crown moldings in photos and you can see the finished product.

Just as an FYI, I did what tupi suggests, although my base does not have a rabbet it is just flush with the wall. Expensive, picky detail, takes fitting the baseboard in a single room from maybe a day to like a week. Very nice detail but pricey and must have a very good drywall/ trim person. Most good wood/millwork suppliers (not the big box stores) will have base and casings with just a simple eased edge. I would not recommend using regular very square-edge stock.

First, I would determine the project design - where does the paneling begin and end? How high does it go? If it goes to the ceiling is there a need to re-install crown? One way to start is to collect as many pictures of these projects as possible and then note aspects that you like or dislike. Do you like more paneling segments or less? Thicker or thinner segments? Do you want any mouldings in the inner panel? I think that any decent woodworking organization should be able to make a scale sketch possibly a simple rendering. You can do the same thing using sketchup or home designer, this is a valuable skill for future projects. For trim work, I would get several quotes. Ask questions about how they are going to assemble and finish and take good notes. You'll surprised at how techniques vary and learn alot about how they work and price their jobs. One way to determine a fair price is to ask how many hours it is going to take. I've done several of these "wainscoting" projects. Done well, they involve pre-asssembling the segments, the joints are assembled with glue and pocket screws so the joint is perfect. Don't allow a carpenter to just nail or glue 1x lumber to the wall. Once the segments are pre-assembled the outside pieces may need to be scribed to inside corners. Please have a look at the four wainscoting videos on Gary Katz youtube site.