Is Mary’s and the saints’ intercession unbiblical?

Not at all. Look closely at what 1 Timothy 2:5 really says: Jesus is the only mediator between God and man. Because Jesus was the God-Man, only he can be the Mediator, the one who is between. Between men and the Father, there is the Son. This doesn’t undercut our belief that the saints in heaven intercede for us because these saints, too, are men; they are members of mankind. Thus, we (men) ask them (men, too) to pray to the one Mediator (Jesus) in order to find favor with the Father.