1- Every BlockChain needs, at least, one "processer" which can be a miner. The processers create and validate blocks (make transactions confirmed). But Multichain is a permissionned BlockChain, then you can create different nodes in giving "mining" right to only one node. Thus, only one node will process the BlockChain.
2- Except if you take the open source code and modify by yourself, I've no know about chossing a different algorithm. But, keep in mind that, MultiChain mining uses low processing. It's a permissionned BlockChain. The processing is adapted to this "permissionned spirit". You can easily create new blocks in less than 15 seconds. And parameters allow you to define the number of empty blocks before to do a pause waiting for new transactions.