Two main things stop a diesel if the battery and starter are OK...Lack of compression and lack of fuel.
You are right about air getting in...if you have an air leak around the filter housing. That does'nt mean you will get a puddle of fuel underneath it...a tiny leak would allow the fuel to drop back to the tank, and that will then of course stop it from starting in the morning.
In the morning before you try to start it.... open the bleed screw on the filter housing, and prime the fuel by hand, until it flows from the bleed screw...then tighten it up.
If it has taken quite a bit of priming, then you know that the fuel has dropped back, and that you have an air leak...then try it without the Easi-start.
If the fuel flows straight away when you prime, then you know that the it's already at the pump and air is not your problem.
If the fuel is at the filter then it should really start without the Easi-start
Old worn out engines like a squirt of Easi-start...
The only way I have found this sort of air leak is to do the opposite of what you might think....I dont look for fuel...I listen for air.
The fuel system needs to have a little pressure in it...get a short length of rubber fuel pipe and fix a schrader valve into it using ajubilee clip(the kind of valve you get in an inner tube ).
The fuel line needs to be parted... near the tank is favourite. Block the pipe leading from the tank...and fit the rubber pipe onto the steel fuel line.
Now just squirt a bit of pressure into it... not too much ...either from a compressor or even a bike pump will do.
Now go along the pipe listening for the hiss of air...it is usually around the filter or even a perished pipe where rubber meets steel...you should also see the tell tale sign of fuel leaking out now.
Good luck...!