The owner's manual for an OUTRAGE 18 gives advice on the drains and whether to close them or leave them open.
If you do not have the owner's manual, I have published a copy of the owner's manual in HTML. You are welcome to read the advice in the owner's manual about drains for an OUTRAGE 18 at
https://continuouswave.com/whaler/refer ... elfBailingRather than try to answer your five questions individually, I will offer some general advice.
As a general rule, in order for the openings in the transom to be drains, their outlets should not be below the waterline of the hull at static trim. In you case, if the drains are below the waterline, then the drains will let the sea flood the engine splash well up to the waterline. If the drains are only one-inch below the waterline then the splashwell will fill with one-inch of water. This is not a particularly worrisome problem. An inch of water in the bottom of the engine splash well is not something to be concerned about.
The aft cockpit sump on Starboard will fill with water if not kept plugged. The height that water will rise in the sump depends on the static trim on the boat. To discover how high the water will rise, unplug the drain and observe what happens.Of course, you should get off the boat so that your weight does not change the boat's trim. If the water rises only to flood just the sump, there is not a particular problem with that--that is how the sump was designed.
If the static trim on the boat permits water to flood the sump and begin to flood the rigging tunnel, more concern is appropriate. The rigging tunnel has a passageway to the fuel tank cavity. If the water rises too high in the rigging tunnel, it will begin to flood the fuel tank cavity. Of course, as more water comes aboard, the boat will begin to sit lower in the water. At some point the process should stop before the cockpit is entirely flooded, as the hull design tends to create more buoyancy as it is pushed deeper into the water.
In some instances, the static trim on the boat can be changed by adding weight at the bow. For example, a cooler close to the bow could be filled with water. A cooler with a capacity of 86-quarts that was filled near to the brim with water will add about 2-lbs per quart, or about 160-lbs. If the cooler is placed as close as possible to the bow, it could raise the static trim at the transom. Another option may be to put some lead weights in the anchor locker when the boat is left unattended. Perhaps 100-lbs of lead in the anchor locker will raise the stern enough to keep the splash well dry and to limit water rise in the sump to just the sump.
If the outcome of the test for an open cockpit sump aft Starboard drain to be left open is good, that is, if only a small volume of water flows and is contained in the sump, then the simplest approach to long-term unattended mooring of the boat will be to leave out all four drain plugs.
If the cockpit sum drain when open begins to flood the rigging tunnel and cockpit, then the leaving that drain open is not a good approach. The drain should be kept closed, and an electrically operating pump should be installed to evacuate water from the cockpit sump.
Test the boat's response to the cockpit drain being open and report the outcome. If the outcome of the test suggests installing a sump pump. you should consider that option. If you decide on a sump pump, its installation has some details about which you may not be particularly aware. The basics of a properly designed automatic sump pump system are laid out in an article in REFERENCE called
Cockpit Sump Pump.
If the boat is kept at a dock and there is electrical power available, the battery that is operating the electrical sump pump can be maintained by a 120-VAC charger, as long as the charger is properly designed and only maintains the battery at the battery manufacturer's recommended float voltage, and as long as the electrical system is not leaking any current into the water.