All bicycle rims have seams. Unless the seam is really offset it should not be a problem when it comes to braking. There needs to be a flat surface on the rim for the brake to set on, When I went and looked at one of these bikes I seem to remember that there was such a surface on the rims. These rims are painted/powder coated and might need to be scuffed up to get the brakes to work well. As for the rear sprocket It should not be all that difficult to get it mounted. My first thoughts would be to try the stock rag coupler and use only 6 bolts skipping every third hole. Or if you don't want to use this type of coupler then one of the aftermarket adapters form Manic Mechanic or sportscarpat will probably work. The rear brake is the standard Chinese copy of the Shimano coaster brake. Have you worked out a starting point for the optimal rear sprocket for a 29 in. wheel yet?
ocscully
ocscully