I would use method #1 and protect the 1' of exposed copper under the floor with a piece of abandoned water hose. 3/8" copper will be fine for this appliance and the amount of pipe involved. Unless the mfg. strictly calls for a larger dimension of pipe, you should be fine.
FWIW, I have about the same length of the same size tubing for my stove (maybe more). It works perfectly.
If you are close to a big-box store, check the electrical section for different clamps. Sometimes, there are more choices in electrical than plumbing. The ones you depicted, though, should work out well.