comma splice exceptions and advanced grammar questions

I'm a high school student and my English class sucks, so...I'm asking here.

I heard that some sentences that look like they have a comma splice are OK even without conjunctions if worded a certain way or used with certain words (I'm not sure.)

I like constructing long sentences because it sounds less choppy so I would really want to know this.

For example:

"She watched the others, one of them looking back, that one grinned."

Usually, in sentences like those, because I wasn't sure, I replaced the 2nd comma with a semicolon as doing that should be grammatically correct. But in these cases, can I just use the comma?

Are these acceptable in formal composition?

~This is my father, that is my uncle.

~Some students find writing easy, some find it excruciatingly difficult.

~It's not a comet, it's a meteor.

~We aren't visiting Pennsylvania this year, we're spending the summer in Florida.



Also, do you have any tips for longer sentences?
I use a semicolon every paragraph and I heard that doing that a lot is discouraged in grammar, especially if I use two or more in a single sentence. But if it's actually a good thing to do that, can I get away with using 5 semicolons in a sentence? What else can I do?

And, I also need better transitions than words like "however, but, also."
For example, I used, "As if that weren't enough, _____" as a transition. It sounds better, but that's all I can think of.

Is it correct to start a sentence with "And" plus a comma?^

I'm feeling very insecure with my grammar so any clarification or advice will be appreciated.