Let's not over complicate this little project. I have seen these open sump holes hundreds of times. I am absolutely sure there are no structural issues here, how long has the house stood without concrete in the hole? There was probably not a concrete floor when it was built and for many years after. If you want to simply keep stuff out and hide the dirt then let's do it simple. Dig out to about a total of 4 inches below your water pipe and valve. Mix up a small bag of Sacrete concrete mix and fill the hole to about 1 inch under the pipe. Float it smooth, you're done. There is no need to get carried away with a special base etc. The size and weight of the concrete patch is negligible as far as settling or shrinking is concerned. Get the stuff and be done with it in an hour!
This part of the foundation is most likely spreading out a roof load from a sizable portion of the sunroom roof. If the soil supporting the foundation has eroded away, this will need to be rectified some time next year.
For now, get a bag of ready mix mortar, the kind you just add water to, it has sand and cement already mixed in in the correct proportion. Mix up a small amount, keep the water to a minimum so it will stay where you put it and not slump down. Patch the hole. That's all this is right now, a temporary patch.
If the hole opens up beyond such that you feel like it's sort of a bottomless pit, cut a short length of something like a paint stick. Thin enough to push through the hole but long enough to wedge inside when turned 90 degrees. Tie a length of wire to the center so you can keep it pulled tight when you push mortar up against it.
As you suspect, water and freeze/thaw cycles chip away at the foundation over time. There was probably once a innocuous hairline crack there at one point. Water got in there and froze, opening the crack. Repeat dozens of times a winter and over the years you get what you see now.
The solution is keep water away from the foundation. I see you have a gutter system over this area. That is good. The patio sloping towards the house is really bad. Not only does it get into foundation cracks and erode the material, water gets under the house structure and can cause uneven settlement, stressing members in ways that was never intended. Some people think crooked floors in old houses are "charming". In reality it is a sign of an unstable structure. Even if the house stays stable, water can be the source of toxic mold. Water has no place in your house other than inside plumbing.
You need to correct the drainage issue. Ideally, it should slope by gravity away from the house at least with a 2% slope. Another solution, not nearly as good, is to place a sump at the lowest point and pump collected water away. I can't emphasize enough how a natural gravity solution is far superior to a mechanical solution. Sometimes you do what you have to do though.
Combine this with the possibility of needing to correct erosion under the foundation. You my be digging up the patio anyway. OTOH, if the foundation is still properly supported and you correct the drainage issue, your little mortar patch could possibly be considered a permanent fix.
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Your best bet is Hydraulic cement. There should be directions on the tin, but it boils down to clean the hole and fill it up. I'm partial to nitrile (or latex) gloves and stuffing it in by hand for something that size. The gloves because some people have skin reactions to cement; also you can throw the gloves away rather than rinsing cement down your drain.
If you do use your fingers to pack it in you'll still want a tool to flatten off the patch. Either a paint stirrer dragged sideways or putty knife should do nicely. It's not exactly the job you need a trowel for.
It sounds like you're right about the pressure, the water should migrate to the drain. If that hole was relieving enough pressure to buckle the floor I'd expect it to be gushing like you'd struck oil.