I tried to drain my electric water heater. After about 30 or so gallons drained out of a 50 gallon tank, the drain clogged from the inside. Turning the cold supply back on didn't help, and now my water heater is full again, the drain won't let water out, and I have to pull out the lower heating element for replacement.

This important part is right at the end, you have a well!! It sounds like you may have run it dry? Check the well and the pump first. The rest is stop valves and pipes to the tank and beyond and not likely to be causing issues. Your sediment issues and problems with the anode is also due to using well water which is nearly always loaded with minerals..

Add a potable expansion tank

turn off gas
turn off cold water to heater
connect hose to spickot
place hose end outside
turn on spickot
open hot water faucets in house
wait.
when water stops, turn on cold water for 30-60 sec or so to try to stir up any sentiment in tank
turn off cold water and let drain.
close spickot
turn on cold water
let fill till water start coming out hot water faucets then turn off hot water faucets.
turn on gas and relight pilot.
finish bleeding air from hot water faucets.
set water heater thermostat

I had the same issue a few months ago. To clear the sediment blow back into the hose making sure the valve at the tank is open. I had to blow two or three times during the draining process. 50 gallon tank took about 45 minutes to drain

Maybe I should look at your unit but, I'm not familiar with an exhaust vent on an electric water heater. That being said, I can't find your system, I did look, if your have 5 gallons of water inside the metal outer tank, I hope it tripped the breaker. If it didn't, you had better turn it off. The only thing you can do really is, pull the inspection panels off, if there is a small piece of insulation covering the thermostats, pull them out. You're going to have to let it drain, air dry. Only the lord knows how long that will take. There is insulation between the fiberglass inner tank and the metal outer tank so, I would have to guess it's pretty saturated with five gallons of water in it. Is this actually a gas heater?

http://www.waterheaterrescue.com/pages/WHRpages/English/Troubleshooting/Tanklets/backflushing.html
The link above gives a list of reasons for the decreased water pressure at your faucet.

As for draining the tank, it sounds like it's clogged with sediment.
http://waterheatertimer.org/Clean-sediment-out-of-water-heater.html

To open a clogged water heater:
Connect a washing-machine-hose to water heater drain valve.
Connect garden-hose to outdoor spigot, and to washing-machine-hose.
Turn on outdoor spigot for 10 seconds and it will open the clogged valve.
http://waterheatertimer.org/Water-heater-will-not-drain.html

For images and product information
For manual


You checked ohms, did you check for 240V across lower element when lower element is turned on?

Energy smart electric water heater has a control box on top of unit. Water heater does not have typical thermostats that can be tested and replaced. Temperature sensors located above each element send information to control box through small black and red wires. Temperature sensor sells for about $40, but there is no test listed that shows how to determine when sensors fail.

Potential problems:
-Temperature sensor is not reading correct temp
-Control box not processing correct temperature information
-Selector on control box set to 2, 3 or 4
-Thermostat temp on control box set lower than 120 degrees F
-Sediment build-up starting to surround lower element so water doesn't circulate (see if drain valve runs full-open

'Rheem 81VP15S point of use' is a single-element, 15 gallon, 120V electric.
Other 'point-of-use' sizes range from 2-1/2 to 6 gallon to 20 gallon.

Electric water heaters have 2 parts: thermostat and element. One is not functioning correctly. Most likely the element caused by sediment build-up.

How to determine if tank has element or thermostat problem:
Do not turn off water going into tank. Put hose on drain valve. Open drain valve. Does water come out like open spigot? If water does not flow, then heater is likely clogged with sediment.

Solution 1: If water flows out like a spigot, the thermostat is probably bad. Read: How to replace thermostat.

Solution 2: Water does not flow, so element is surrounded by sediment build-up and cannot heat water. You need to clean out tank with shop vac, and put in new element ... plan full day and 3 trips to hardware store. Simply draining tank will not remove sediment. Sediment is chunky and will not come out voluntarily.

Buy element before starting:
-Turn off electricity
-Open cover on side of tank, remove insulation (these have to be put back for proper operation of thermostat)
-The are 2 electrical parts >>the element and the thermostat which is located above element
-Element wattage is printed on element and also shows on label on side of tank
-I think Rheem element is held in place with 4 hex bolts
-Take photo of element to hardware store so you have part on hand before removing old element
-If element is 1-1/2' diameter hex type, you need inexpensive element wrench from hardware store + the element

Drain tank and clean sediment:
-Read: To open drain on water heater so water comes out
-Read: How to clean sediment out of water heater

If it is leaking from the tank, you'll have to replace the heater.

the hot water tank may be filled with scale buildup on the inside. drain a few gallons out of tank until the water runs clear. it is best to drain out water heater at least 2 times a year.