TLDR: Water conditions/tank maintenance/setup are all ideal and stable. Mollies and pleco healthy. FOUR guppies have died, (one after the other, not at the same time!) and the 5th guppy is in bad shape. Are my mollies murdering them? Is my Molly pregnant?

As far as I have researched, I am doing everything that I am supposed to, but my guppies keep dying. I have had 4 die over the last 2 months. I have one left in there, as of today his fins are clamped and he is barely moving. The mollies and pleco are totally fine and have been in the tank since day one! Something is happening that is only affecting the guppies. Please help me figure out what I am overlooking.

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Here are all the details I know I will be asked for:

TANK SETUP:
10-gallon tank. Cycled it before I added any fish.
I currently have 2 mollies, 1 guppy, and a pleco.
Four guppies have died, but they were not in there at the same time. It happened one at a time over the last 2 months.
The most fish I ever had in there at once was 2 guppies, 2 mollies, and a pleco.
Filter changed monthly
I have a heater and a separate thermometer. I check the temp daily and the water has stayed around 75 up until this last week as the weather changed. It was at 69 this morning so I turned up the heater a little and it's now at 75. (Temperature could only have been a factor in the most recent death, not the first 3.)
Automatic LED lighting

WATER QUALITY:
I check my water levels DAILY.
Nitrites are CONSISTENTLY at 0.
Ammonia is usually close to 0, and has never once been above 20, but when it starts to creep up I do water changes to lower it.
Ph is almost always at 7.5, although today it was at 8.
I started off using Aqueon Water Conditioner, for the last few weeks have been using Prime, as I saw it recommended in another thread.
I have added aquarium salt with water changes.
As mentioned above, water temps have been stable.
I feed my fish once per day. In the beginning, I was feeding twice per day, but after losing multiple guppies I dialed back to once per day to try to eliminate the possible factor of overfeeding.
I can't see anything visually wrong with the water. It is crystal clear. I haven't had any issues with foggy water, algae, or anything else.
I don't see any physical signs of parasites or infection. Guppies are fine one day and dead the next.
When I do water changes, I use a gravel vacuum and add new water with prime and aquarium salt at the recommended levels.
I usually do 25% water changes, but once or twice I have done 50% - after there had been a dead fish in there. I have done water changes once per week, with few exceptions, but during those times I was still checking water daily and all my levels were in a good range.

TANK DRAMA:
I was told that mollies were not aggressive and could be in a tank with guppies. However, I noticed that one of my mollies was majorly bullying the guppies and nipping them. After this observation, I assumed that this bullying must have been what lead to the deaths of the first 3. I was extremely frustrated, as I was told that these fish would be fine together, so I did more research on mollies. I determined that I have 1 male and 1 female Molly, and the FEMALE was the one being the most aggressive. (I have seen the male chase the guppies around but hadn't noticed any nipping or charging.) Female Molly does look a little bit "fat", so I thought it was a possibility that she was pregnant and that was causing the aggression. I got a tank divider and I separated the female molly from all the other fish. (If the female is in fact pregnant, she should be giving birth by now because I have had her for almost 2 months????). I thought that this separation would keep the guppies safe and that they were OK with the male still being on their side. Today (more than a week after separating the female), I woke up to one dead guppy and one visibly stressed guppy who I think its about to die.

MY GUESS:
I am operating under the assumption that for whatever reason both of my mollies are aggressive and are killing the guppies. I have come to this conclusion because of the fact that the mollies have always been healthy, and that these guppies have died one-by-one over the course of 2 months, and some of them were never even in the tank at the same time, so I know that them dying is not a result of some undetected nitrite spike or some other isolated event. It has to be something that is a consistent factor in my tank. Because of this, this morning I switched the guppy and the female molly so that the guppy is alone on the other side of the divider and can't be harassed. But as I already said, I have never SEEN the male being super aggressive and everything I've read says they shouldn't be as long as they aren't overcrowded which they haven't been, which makes me wonder if I am missing something.

I'm at a frikkin loss. Thanks in advance from a sad fish owner.

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pregnant? and if so, why has she not given birth after 2 months