I use ether /cold start on my 2 diesel Ford trucks so as my merc too.
My marine diesel mech fnd says is ok, I not sure what is the real issues, have heard Cold start do damage engines.

Or change both fuel filter, sounds like more of the glow plugs not firing, u can get voltage, if u have a volt /ohmteter test the GP itseilf, u need to undo the wire. If its an open circuit then it F*cked. The GP has either high or low resistance, open circuit def not joined at all.
Good Luck.

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