No, you can configure a single node to be both a miner and a witness. Since there are already a lot more detailed tutorials about that, this post was specifically intended for multi-node setups, because that's what seems to confuse most people.
And no, you don't have to run a seed node to mine. Seed nodes are for bootstrapping new nodes in a distributed network - they are specific initial nodes to talk to when you join the network, that are known to be already part of the network, so they can tell your new node more about who else is currently in the network.