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I would spray the threads with "Kroil" or any penetrating oil numerous times over a few days before you try to remove that pigtail. Let the penetrating oil sit for hours to allow it to do it's magic. Then use a 6" or 8" Ridgid pipe wrench, putting it as close to the low water control as possible to reduce the chance of breaking the pigtail. Try turning the pigtail just slightly CW as if you are trying to tighten it before you try to remove it. The last torque that pigtail saw was when it was installed so the resistance to turn will be the least when turning it CW as if still tightening it. When you get it out, replace it with a brass pigtail and use Permatex anti seize lubricant.

The farther you grab it from where it screws into the LWCO the more chance it will break off.

Oil Burner service contracts rarely include ALL the necessary maintenance needed for a steam boiler. I used to do those "clean-outs" when I worked on oil burners in the 1970s and 80s. The important thing about the steam boiler is to make sure you have good water quality, the LWCO works and the pressure switch works. Part of that is to be sure the pigtail is not blocked.

Next, every few years, (every year is better but that never seems to happen) you want to flush out the wet returns and the bottom of the boiler. There is a lot of mud that forms at the lower sections of the system that can block the condensate from returning to the boiler. Now testing the limit operation (pressure and low water) and the safety control (with the reset button) are part of the oil burner clean-out… the cleaning of the mud from the bottom of the boiler and the pigtail are not included. The oil company would call that part of the maintenance "a plumbers job", since that kind of work can get involved. It is not included with oil burner that is covered by the service agreement.

That said, Our oil company could do the work if you wanted it done, but there was an extra charge. So that is why it was not recommended every year. Maybe every 3 to 5 years, unless something was discovered that caused a concern, like the LWCO was getting stuck…

When you said "…and drain out and replace some dirty water every few weeks." I hope you mean that you were letting the dirty water out of the LWCO using the valve at the bottom of the triangle. That valve should be open when the burner is operating and there is some small amount of steam pressure. When you open the valve, the burner should stop, the water should flow until clear water is visible, and steam exits the valve. This does 2 things: 1. ensures the LWCO float is not blocked and will turn off the burner if the water gets too low, and 2. prooves the electrical switch works when the burner stops 

I hope this info helps a little

Scrape the pigtail outside with a file or a knife to see if it is brass or steel. Steel will be shiny brite and brass will be yellowish.

If steel it should be replaced with brass.

If it is steel it could be rusted in place and may be difficult to remove and it could break.

Best bet is a small pipe wrench grip it as close to the boiler as possible. If it feels like it will break you may be better off calling for service.

Pigtails plugging up can be a common issue.

Also you should not have to reset your burner that is not normal operation and an indication something is wrong.

Depends a lot on whether the system was pulling from the boiler or the wet return. If it's pulling from the boiler such a system can work very well, although ideally the return will be to a tapping some distance from the supply.

Probably not picking up much heat if the baseboard supply and return are near each other, but they do need to be below the waterline.