Questioning grammar

In general, I don’t think I have too many grammatical issues with my writing. Before I decided to return to college to complete a master’s degree and teaching certificate, I was a copy editor at a newspaper. I loved that job. I loved correcting other people’s writing. I loved finding typos and homophone errors and overused hyphens. It wasn’t schadenfreude. It was about making everything look neat and clean. The challenge of learning to design pages was another fantastic part of the job. It was like playing Tetris mixed with Cranium all day long. But, it wasn’t challenging enough. I think that’s my biggest problem with writing. It seems less like a challenge and more like a tedious task. 

Despite the fact I was an editor, I still make my own grammatical errors. That’s why professional writers have editors. Even editors need editors. A fresh eye can see what the writer often cannot. In looking over my writing, I don’t see any obvious patterns of error. But then again, they may not be obvious to me because I simply don’t recognize them.

I’d like to learn a couple of things that have been bothering me lately. I have avoided using semi-colons in much of my writing, mostly because I don’t use them enough to feel comfortable with them. It’s a vicious cycle. Here's an example: 

Eating this chickpea and arugula salad has the opposite mental effect from eating a cheese dog covered in ketchup. Instead of a wee bit of self-loathing mixed with Homer Simpson-esque drooling, you feel self-satisfied and healthy and maybe downright proud of yourself. That’s OK for a short while; you’re not a saint (not, yet, at least).

(Please forgive the OK for okay. That's AP style, which was the style in which this was written.)
Is that correct? I think it is. I hope it is, since it was published that way. When is a semi-colon absolutely necessary? When does it make a sentence look better? When it inappropriate? Is it correct to write: He went to the store; however he found it to be closed?

My next question is on certain uses of -ly. For the most part, I understand when to use -ly correctly (no pun intended). There are times, though, when I waver.

For example:

The perfect cinnamon roll is an elusive one. It’s fully cooked, but gooey, bursting with cinnamon and most important, not dry.

I tend to want to add an -ly to the phrase "most important.” Which is correct? I can't think of another example for this one.

Finally, I have a who/whom question.When should I use whom? Is it proper to say, "to whom does this belong?" Is it really necessary these days? It seems so stuffy, snobby, prudish, English. Here’s an example in a recent piece of my writing: 

Often, it is taught only in select classrooms with students whom teachers deem ready for such high-level thinking.

In that case, I used whom. I didn’t consult my style manual and didn’t even bother to look it up online. I was so cavalier with my grammar, even in this literature review that forms the base for my entire master’s project. When I used whom, in this case, I felt like a renegade, a silly child, a fraud. I really wasn’t sure whether it was correct. Worse yet, I knew I wasn’t sure! Yet, I still didn’t bother to figure it out. I re-read this piece of writing, and I want to know, without a doubt, when to use whom. I want to know whether to cringe or cheer.

There you go. There are my questions, my concerns, my thoughts, my hopes, my dreams, at least as far as grammar goes.