Here's some thoughts for you:
If you are pumping 13 gallons per minute at 18 psi then you will pump less gallons per minute at 50 psi, that's just the way centrifugal pumps work. If you feel like it you can throttle the hoses back until your system pressure stabilizes at 50 psi and see what your flow is at that pressure. That will tell you more about what your current pump is doing at the pressures you typically run it. If you use hose shut offs with additional lengths of hose downstream from them you can do this without spraying the water out of the ends of the hoses, it's easier to fill buckets this way.
That is a split top seal on the well, and luckily for you, it is a double pipe seal. Try putting a channel lock pliers on that little pipe cap on the top of the well seal next to your drop pipe and see if you can't wiggle it up and out of there. Chances are there is nothing under it but a short nipple and it will come out pretty easily. If this works you can get access to the inside of the well pretty easily this way.
I don't want to be rude or overly emphatic, so how do I say this? I mean, I really don't want to be too much of a mean old man, or ruin your fun, etc., etc., but here goes:
Do not try to take the well seal out at this point. Not unless you have a secure, and I mean secure, as in won't come down on your head, ever, way of lifting the entire drop pipe, because that is what you have to do to get the well seal out.
You might call this rule number one. Do not rig up any kind of half baked tripod or half baked anything and inadvertently bring it down on yourself or others. A few hundred feet of drop pipe filled with water is heavy > so treat it with due respect and caution and don't get hurt. Rule number one.
Another thing not to do:
Do not drop the pump and the drop pipe down the well. Just don't.
You might call this rule number two. As in rule number one, a drop pipe that long, especially full of water is heavy and should you find yourself watching it disappear down the well at 32 ft./sec./sec. you will be very unhappy. I would say, "trust me I've been there", but actually, I havn't, I've done many many, submersible removals and installs and I've never dropped one, knock, knock, knock on wood.
Okay, got that out of the way. I guess that wasn't so bad. Please, don't hurt yourself and don't drop it.
Let's see, where were we? If you can get that second pipe out, not the drop pipe, the best thing to do would be to see what the water level is. A few things you might try are: Put a weight, thin enough to get through the hole, on a string and with the pump off, run it down until you hear it sploosh in the water. Mark the string, and measure it as you pull it up. Sound travels easily up a well and if you can get a weight through that opening, and the weight is nice and flat on the bottom so it makes noise when it hits the water, you will likely hear it. Another thing you might try is using a fairly long spool of small two strand wire with the ends exposed in some kind of weighted end piece and an ohm meter taped to the spool and connected to the other ends of the wires. An ohm meter with an audible continuity tone will make this easier. It all depends on what you have on hand and what you like to work with. The only thing to really not do is, you guessed it, drop the stuff you are using down the well.
If you can do this successfully and you feel so inclined you can try to get a reading after pumping the well for a while. If you can get this reading, after say, an hour of pumping, you will have some idea of whether the well has significant drawdown. Since this will be harder to do than the first measurement, unless you use a contact probe, you may also want to consider the following procedure: Let the well sit for an hour at least, no water use from it. Then do a flow measurement, like you did at 18 psi, or some low pressure so that your flow is maximized. Be as accurate as you can. Let the well run flat out for an hour and do another flow measurement at the same pressure, using all the same methods as in the first flow test. If there is significant drawdown your flow after some time will be less because the pump will be lifting the water further and so it will pump less water. You may want to do this after longer than an hour, say four hours of running. If there is a difference, this test will not tell you how much drawdown you have but if the flows are very similar it will tell you that you have very little drawdown at that flow. If that is the case, and you are going to pumping it at a similar rate to the rate used in the test, you should be golden.
I would not recommend that you try to figure out where the pump is, as you correctly state, it will not matter when it comes to how far it has to lift the water. When you pull the old one you will find out where it is, just put it about the same place. Trying to find the pump with a weight on a string will mostly be either futile or you might just end up snagging the weight and having to try to retrieve it when you pull the pump.
Getting some idea of the pump level and drawdown will tell you a lot about what pump you will want to use. Bottom line.
Also though, just for fun, if you can find the pump starter box (there is one somewhere because you have three wires going down the well), see if it lists the horsepower of the existing pump. That will tell you something too but off the top of my head you will be hard pressed to find many 300 foot wells making 13 gallons per minute with 1/2 horsepower pumps in them. More likely what is in there is a 1 to 2 horsepower pump.
Okay, enough for now. You are on the right track. Be careful.
Best,
Damian
