First off, great pics to try and help sort this out, answers a lot of questions that you didn't address with words.

The deposits on the #1 spark plug looks more like a fuel impurity caused deposit more than water, so I'm not sure that the water temp shooting up and the spark plugs are related. It doesn't make good sense on why it would only be on one spark plug if it was fuel related though, unless you have a single injector that is slightly clogged, which is entirely possible.

As for the deposits in the radiator cap, it appears you're over due for a strong cooling system flush and fill, because to me, it looks like some calcium or lime deposits, which after a while can form hard pieces that can block your cooling system ports pretty hard, and shoot up in temp as you described. After you get the whole system flushed DO NOT USE TAP WATER WHEN CUTTING YOUR ANTI-FREEZE 50/50, USE DISTILLED WATER. Distilled water won't have the calcium or lime, or any impurities to bring your problem back. Some places have a water works that does a good enough job that you can get away with using tap water, your locality is obviously not one of them.

As for the possible warped head, watch for fluid consumption, and pull a compression test.