Prefacing all of my post with I have never made a mod before for the Sims, I have never used Blender before other than for this, and by and large have been following tutorials to try and get by. I wanted to try my hand at separating the coat from the Business and Hobbies Fur Coat full body mesh, and turning it into an accessory coat as it filled the need I had for a Maxis Match kind of shabby fur coat. I followed these tutorials (this one for a coat/jacket separation, and this one for general clothing separation). I ended up getting to where the mesh was isolated on its own but got stuck on the steps of trying to take the midsection from another mesh by EA to make it a full shirt properly, but ran into a lot of issues (namely that when trying to use this shirt from the Simtimates kit, I couldn't get rid of the lapels where it was poking through the coat without well, removing from the midsection underneath (image link here ), and couldn't try to shrink it down as it was too obvious. There weren't really any shirts for the feminine frame that I could find that would be wider in the center of the body/wasn't a bra, which obviously I was unable to use due to not really knowing how to remove the clothes but not the body underneath if that makes sense! Here's what the mesh looks like with nothing underneath (feet are there, just cropped out of the image). I'm sorry if this is very obvious answers to my questions, I just genuinely am at my wits end and have tried many other shirts, even shrinking down a bracelet to see if that would keep the torso there but not intersect (which was kind of obvious it wouldn't work, but to be fair, I was very exhausted when trying to work with this). Obviously, this is a much more advanced project than I should have taken on (I do not even want to try and think about trying to get it as an accessory top, as that seems even more complicated to be honest) but I would love any advice or anything that is a "oh duh!" moment that I missed out on due to inexperience! Thanks !

If your original intention is to create an accessory top that can be worn in combination with other tops then you shouldn't be including any of the body at all on the final mesh. You would only need that for the weight transfer.
The first tutorial you linked to for seperating a jacket doesn't cover any uv remapping or texture altering that might be needed. You can even see in the finished item in the video example that the texture for the jacket is also painted onto the torso underneath because the mapping wasn't altered.

Thank you so much! Are there any tutorials you know of to cover UV remapping/texture altering or anything that would help in order to get this as an accessory top, or any that would explain this process of what exactly I need to do to turn the isolated coat into the accessory version? I really appreciate your reply!

I'm honestly having trouble finding any tutorials specifically for this kind of creation. Which surprises me because there are so many accessory jackets out there.
I would highly suggest heading to the official Sims4Studio forums and reading through some of the tutorials there relating to the different parts of clothing making. It helps to fully understand the basics of how stuff normally works so that you understand in what ways you can deviate from that to make what you want.
You can always start your own thread to ask questions if you get stuck.

I fully agree, I really expected there to be a ton of tutorials outside of just one short video for it that wasn't super clear. I definitely will do that and look into other tutorials, so I really appreciate your help with that and to give me ideas of what terms to look into! Thank you so much again :D