The powertube is not meant to come off. If it does, and the valve has any age to it, which it does, you definitely need to replace the red oring, which in your case appears to be missing anyway. Its not a standard part since the powertube is not ever meant to come off so you will have to order it as a special order item from one of our vendors here.
Your bolt sounds like it is a level 7 foamy bolt, which was standard for AIR valves. If the foamy is missing, you can get a new one and glue it on. Make sure you clean the metal really well to remove any residue. Some glues can eat away at the neoprene over time, so use a good quality epoxy when gluing the new foamy on.
If you have the spare cash, get a level 10 bolt.
Check the condition of the sear. A worn sear can cause the bolt to sit farther forward so that the bolt stem doesn't seal against the powertube oring. You can use a shorter powertube spacer to compensate, but if the sear is worn, you should probably be ready to replace it.
If you need a shorter spacer, you can sand down the spacer that is installed now. Place a piece of sandpaper on a flat surface such as a table top, and slide the end of the spacer back and forth across it until you remove the desired amount of metal. If you later need a longer spacer, add a level 10 shim between the powertube tip and the spacer to increase the spacer size by 0.010" in length.
Don't worry about the oring on the powertube tip. It is only there to keep the tip from vibrating off the end of the powertube.
On a level 7 mag, when you fire the gun and hold the trigger, there should be no air passing through the on-off. If it does, then there is a leak that should be addressed. The on-off always has the same amount of air pressure applied, it doesn't build up pressure unless the regulator isn't working. If you replaced the regulator seat oring, this shouldn't be a problem.
Any light synthetic lubricant should work fine. All that really happens when you put in too much oil, is you prevent the orings from sealing against the metal surface. The excess liquid will be blown out in just a few cycles so it shouldn't be an ongoing issue. The biggest headache it creates is if you have a barrel on the gun when there is too much oil in the valve. It coats the breach and barrel with oil and makes the gun very inaccurate.
Any gurgling or liquid sound in the valve is most likely an oring that is not sealing properly. You may have to take the valve apart and clean it again to make sure there isn't a piece of dirt caught in something.
Make sure you replace the bolt spring. It is just as important as the orings and is a consumable item that wears out.
Except for the Automag in front, its usually the man behind the equipment that counts.