Consequently, the "one mediator" is perhaps richer in meaning than it appears to be. This verse is only relevant ONLY if it is never right to ask someone to pray for you.
Or, I suppose some kind of tehory could be advanced that it is okay to have an earthly mediator, but not one who is in heaven. But I don't think that can be found in the verse or anywhere in scripture.
Summoning the dead is very different from summoning the living (1 Sam. 28), and we are prohibited to seek them out (Deut. 18:9-14). As far as mediator is concerned, there is a more fleshed out meaning (Hbr 8:6) which places Jesus as the sole (Jewish-style) priesthood, but this just amplifies Jesus' position.
We already have one in heaven. His name is Jesus.