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Don't worry about the pitch, it doesn't matter as long as it's reasonable level.

But since you hear gurgling, there is likely a "settling" that has occurred in your house that has changed the pitch of the pipe under the floor that supplies that radiator. You replaced the valves which rules out another cause which is the valve's seal plate thing rusting and dropping from the valve's vertical screw.

If you hear a radiator gurgling try raising the radiator (both sides) a small amount every few days, like 1/4"-1/2" and see if the behavior is affected. If you are lucky, you will correct the pitch enough to let the condensate drain out of that section of pipe.

As it seems like you know, the reason it does this in the shoulder seasons is the trapped pool of water has time to cool off enough that it causes the steam to collapse when they come into contact. When the water is warm/hot the steam can co-exist with the water.

You can use a two-by-four and a block of wood to make a lever and you can use 1/4" pieces of plywood or whatever under the legs.

Main vent sizing is dictated by the amount of air in the main and the steaming rate of your boiler. A boiler that takes ten minutes to produce steam for a main doesn't need that main vented in two minutes. A 40-foot 2" main can usually quite effectively be vented with one or two Gorton #1 vents.

A properly sized boiler will only turn off on pressure on the coldest "design day" days, hardly justifying the added expense of a Vaporstat. Here in New York that's perhaps once or twice a year.

Unless the entire system is evaluated, and each repair assessed and prioritized, throwing parts at it often wastes money that could be better spent correcting the worst deficiencies. Amateur assessments often default to the easy stuff and miss the true problems.

low pressure covers many sins