I try to answer to your questions as good as I know
1) VALMET didn't make any S-by-S shotguns. The first model was 16. cal. LEIJONA in 1948. In 1950 came cal. 12.
2) The O/U design based on the french Damon Petrik, St.Etienne. The main designer of VALMET Leijona was tecnician Paavo Narinen.
3) Yes they did. :-)