We all know Roadmasters and Huffys are the HiPoints of the bike world. Pacifics too. Like the HiPoint, they do do what they were designed for. I would put the derailure back on the outside of the frame. Any so called upgrade can be found on other ditched or trashed wally world bikes. The bike, believe it or not, judging by its parts is no older than 8 years old. I actually have a slightly older Roadmaster Mt. Fury at home waiting for a rebuild. Because I can work magic on crappy bikes.
If I were you I would be looking for the cheapo bikes with better components to use such as the Next bikes. If you want to keep it hardtail; find a full suspension Next, the forks are a tad stiffer. Pacifics tend to use the same forks as roadmasters. Better yet some Mongoose forks, especially if it has an Element fork act as if they are progressive rate. Next would be some thumb shifters or trigger shifters, both can be found on cheao throw a way bikes. Then you can squeeze in an aluminum seven speed wheel from one. There are no other ratioed 1pc cranks though for mtbs.
You wany to lose weight, you will with that bike. The users here can say what they want about the cheap bikes, I dont like them either, but I can make them work. Any good lbs or co-op will work on any bike- even if its with a grit on their face. Most people are going to have dept store bikes than not anyway. It don't take anything but time to polish a turd. All "upgrades" can be found for free with patience.
Last thing is I would appologize to my mom for being any kind of rude and ungreatful. She got you a bike because you needed one. I dont know how well off she is, but she did you a favor she really didnt need to do, spending money she really didnt need to spend. Just to make you happy. None of use want to be given a crappy bike, but if my mama gave me a bike because I needed one- even if it was a first gen Huffy Stone Mountain with bent side pulls, I would proudly take it as if it were a 10000 dollar roadbike because shes my mama and all shes concerned about is me having a bike I need, not what kind it is.