1. "under protest" means that he did not agree with the fine and complained about it. To protest something means to take action or statement against something that expresses disapproval. 2. "unwillingly" means that he was against paying the fine but had to do it anyways for one reason or another. 3. "reluctantly" here means the same as "unwilling", but the sentiment is not as strong.

I understand what you said, but the sentences below sound more natural. My glutes are sore from squatting. My leg muscles are sore from squatting.

"You haven't had a job since then?" I believe "haven't had since" is correct because it implies a continuous state of something happening. If you Google each phrase in quotes, you'll see there are only 8 instances of "didn't have since" being used on the internet. There are 341,000 for "haven't had since". (You need to use quotation marks to do an exact match search though.)

Japanese Market Entry Strategies for Chinese Companies ~A Case Study on Xiaomi~ The subtitle could be expanded like this (if this is what your paper is about). ~A Case Study on how Xiaomi Overcame Japanese Market Barriers~