No problem! I'm glad you found my post useful. I was worried I was coming off too harsh at certain points.
This is a great mindset to have. I was just spit-balling some themes you could focus on. You don't have to shove these themes in the readers' face if you're not comfortable with it. As long as you have a theme in mind, it will help guide your story and give a valuable message at the end. Hinting at the themes in the earlier episodes would be a great way of introducing it without appearing too blunt about it. For example, a man could make a rude remark about interracial/homosexual couples in one episode, and Cory reacts negatively to it. This can be an entire scene, or just a quick interaction depending on how much you want the reader to focus on it.
Personally, I would recommend rethinking the character roles for each supporting member. Instead of having 5 separate characters with extremely similar roles in the story, try combining multiple characters into one single character. Maybe Joy and Isa could be combined into one character. Consider grouping each character into an archetype. If two or more characters fall under the same archetype, it's best to merge them into one character. Storytelling is all about simplifying your message so you can tell your story quickly and efficiently to the reader. If certain characters don't advance your story in any way, it's best to narrow down your cast to the ones that do. When you're confident with each supporting character's role, introducing them into the story will feel more natural considering how you've fully fleshed out their purpose.
Your latest episodes certainly have better shading than the first few episodes. It looks like you've stopped relying on airbrush shadows and have begun to familiarize yourself with hard shadows. However, you still lack strong shadows. For example, when Cory and Emmet meet on the balcony(?), we assume it's nighttime. However, the warm colors and simple shading make it hard to tell that its nighttime. Here is an example where I noted the color difference and lighting change. In the original panel, the light source seems to be coming from the top left, but considering that there's a building right in front of them, it's impossible for the light to hit them at that angle. I corrected the light source to be right on top of them, creating a more drastic lighting angle.
Really, all you need to do is familiarize yourself with different light angles and play around with color. If it's a sunny day outside, use warm colors (red, orange, yellow). If it's a cold night, use cool colors (green, blue, purple). Try to be more confident with your shadows and use them all over the body when needed, not just under the neck and clothing folds. A few tutorials can go a long way!
This is a tricky question that's hard to answer. It depends how much you alter the model, really. I've seen people who merely trace over the image and change the colors a bit, and they seem to be doing fine. However, that may not be enough to be considered legally altered. In reality, the chances of a SketchUp representative hunting down a small time webcomic creator and suing them over a technicality is incredibly small, but I prefer to play it safe. If you want to take the risk, then feel free! I would also advise not using Sims for backgrounds. There is no license to purchase in order to use it commercially and EA is protective about their IPs. Don't use Sims unless it's a personal project with no money involved.
You said you use CSP, which is great for creating backgrounds! Their library of backgrounds is actually astounding. They're very good quality and a lot of them are free. If you can't find the specific background you're looking for, try using the perspective ruler and creating you're own. It's incredibly easy because it snaps your lines to a perspective grid while drawing. You can whip up a fully polished background in 30 mins - 2 hours depending on how complex the set is.
Ignore people who call it "cheating." All professional artists use shortcuts and models to get their work done faster. As long as you're not using models without permission, you are 100% fine with using them in your work. All CSP assets are free to use both personally and commercially. If you don't want to pay for SketchUp licenses, I fully recommend using CSP assets to your advantage. While I wouldn't recommend tracing over the CSP human models (they can look weird and uncanny), I would certainly recommend using them as reference if you can't find the specific pose you're looking for.