Mediums and seances aren't necessary; people attempt to communicate with the dead all the time without an intermediary.
Then that wouldn't be spiritualism. Spiritualism is, by definition, the belief that the dead speak to the living by means of a medium or a clairvoyant of some sort and/or through a seance. You say that praying to the saints is spiritualism, but that is not so.
Nonsense.
Of course it isn't nonsense. Prayer is only worship if it is accompanied by actions or words which are themselves matters of worship. The reason people think things like prayer are worship is because they, first, have ceased to actually carry out definitive acts of worship, and secondly, they have formed an inadequate and unchristian understanding of life after death. The former I have discussed, and the latter is reflected in thinking God is not the God of the living. Some people have a problem understanding that being a Christian doesn't stop with physical death. Most Protestants have prayer chains and circles and ask one another for prayer all the time, and they never think of it as mediation or intercession which infringes on what our Lord does. But, if that person passes on to be with the Lord they insist that one can no longer ask them for intercession, and if they did then it is somehow idolatrous and infringes on what is due only to Christ. Silly. Christians pray for one another, and ask each other to, and physical death is nothing at all to that. Not for a Christian.
Not so, even though you seem to think repitition makes it so.
Oftentimes a person takes it on himself/herself to communicate with the dead. An intermediary in not necessary for spiritualism to take place.