1. It wouldn't be the first time I've seen a M39 cleaning rod used in a M28/30 and with this one being so late, I suppose it may be considered correct but I'm sure those who know more than I do will chime in on that.
2. A M28/30 muzzle cover would be the most correct.
3. From what I remember, a Sk.Y marked canvas/leather sling is correct on these, however considering the Finns used anything they could, any Finnish sling could be considered correct.
4. The buttplate is fine. I'm fairly certain they were no longer stamped with the last two digits of the serial in 1940. I can't speak to the HV mark though.
Hope that helps some. I'm sure others will chime in soon to help more and correct me if I made any mistakes.