Yes, you can avoid eating spicy foods. There are mainly three types of solutions in avoiding spicy foods in South Korea.
Some of the foods there are not spicy as much. These include Samgyetang, Kalguksu, Seolleongtang, etc... Kimuchi is always served as a side dish along with some more, but you can leave them untouched.
Almost all waiters in local restaurants don't speak even a basic English. In my experiences learning a basic Korean won't get you much rewards since you don't understand their response. For example suppose that you say you don't like spicy food, and a waiter said something like "This is not spicy so do you try?". How can you know what the waiter is talking about? Add to it the fact that Korean language, an isolated group of language families, is notoriously difficult for any speakers around the world.
Fortunately many restaurants in touristy places display photos as well as English descriptions in its menu, so it should be a decent clue for you. I may recommend that you memorize some non-spicy foods in advance, in Korean as well as English.
South Korea is full of cafes (including Starbucks). Especially Seoul has arguably more cafes than any cities in the world. Many cafes are open after midnight in hot places such as Hongdae, Gangnam, and Dongdaemun so you can eat even late at night. You can order sandwitches, pancakes, a variety of cakes, waffles, breads, even salads, pastas and lasagnas (but rare) but some of them may be too sweet for everyday meal (actually I do, though).
Like China and Japan, South Korea is full of convenience stores. It is clear to see which food is spicy from the packages.
Since you know much about Japan, I will share my tips in regards to the difference between the two.
Unlike Japan, where leaving food may not be appreciated by some people, you should feel free to leave the food in Korea.
Most people in both countries speak little or no English, but while most Japanese people try to understand your English with smirking faces, many Korean may not reply in English and instead speak Korean so fast. Some might even ignore you. This would rarely happen in touristy places like Myeongdong, Dongdaemun, or Garosugil but did so often on me around the Gangnam station and the entire Incheon.
Compared to Japan, especially Tokyo, there are far fewer Western restaurants in Korea; most are local Korean restaurants with some, but not many, Japanese ones. There exist some Chinese, Vietnamese, Thai, and Italian restaurants but you shall search for them in advance as just walking across the street won't get you any of them easily.
The acceptance of a credit card should be 100% with the government regulation, so you can just hand in your card and then don't need to bother to care about the charge.