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Not directly, no. If you pull from the side of the furnace, it will first drain the first slot and then attempt to drain the second. You can't change the order.
I think the problem the OP writes about is that it doesn't get destroyed, even when you try to wrench it. I also had that once with a Quantum Chest, it said "0 of (some item)" and was unbreakable. Someone with creative mode had to destroy it.
If you use machine blocks as decorations, sure. ;)
But they are also used to prevent cables connecting to the machines, which allows for more compact setups.
Try http://www.minecraftwiki.net/wiki/Command_block#Arguments
I think it should be /xp -15L @p[lm=15]. But I am not sure whether it just ignores that player then and then chooses the next matching one.