So my install says rookie all over it.
As long as you are pumping away from expansion tank you should be ok with the pump on the return side of the boiler. But by the looks of the system you seem to be pumping into the expansion tank.
The boiler feed only needs to be connected below the spiro-vent to help eliminate any air the is introduced into the system a little quicker. It's just the preferred method.
The drip in the venting could be condensing flue gases. This would not be a good thing and would most likely be caused by to low of return temps (130-140 min). Once this happens it just get worse and worse because your heat exchanger will start to get plugged up. The flue gases condense and then trap particles in the flue gases.
Or you could have a failed rain cap on the top of the vent and you're getting rain water in the venting!
The atmospheric port on the BFP will drip when there is a sudden change in pressures. IE if the boiler system pressure jumps quickly and the house pressure drops when someone opens a faucet. Especially where you have it piped on the supply of the boiler (hottest water temps). You're also pumping into the backflow preventer which isn't helping.
Move the boiler feed to tie in under the expansion tank and repipe the system so that you are pumping away from the expansion tank.