Rats have twice not seated certified winners (NH senate race and Indianna house race). I am not sure why the Minnesota situation is different. The Senate is not guided by state law. Coleman will never be seated. I am not sure if Republicans can stop the seating of Franken if Coleman is certified as the winner.
In 1968, in a nearly unanimous opinion by Chief Justice Earl Warren for eight members of the Court, the Supreme Court held that "the Constitution leaves the House without authority to exclude any person, duly elected by his constituents, who meets all the requirements for membership expressly prescribed in the Constitution." Those qualification are expressly enumerated within the body of the Constitution and relate only to eligibility requirements such as age and residence.
In answer to your second question as to whether Republicans could stop Franken from being seated if he was certified the answer, according to the Constitution and the USSC, is an unequivocal NO!! This also applies to Coleman should he win his suit and become certified by the Minn Sec of State.