In full golf stroke I have no problem (in fact I like) to see loets of shaft lean, even if it means a little "arch" in the left wrist. So your understanding of having the shaft lean forward a bit to take loft off works on 90 percent of the shots you hit.
In putting, the loft/roll relationship cannont be overlooked. If you have zero loft on your putter, then you had better have a malet type head that is very deep in length and not deep in face. This head type will move the center of gravity far enough back to give some "effective loft."
Standard loft is roughly four degrees. This number was created by R&D departments of manufactureres. They find that four degrees of loft rolls the ball "better" than other lofts. Now again this is not uniform because of the design characteristics of the putter head but it is a good "standard" to go by because most all putters I see come in here are 2 ball, answer types, and traditional blades.
So, by leaning the shaft forward you are taking off loft. Just the other day I gave a lesson to a teaching pro and he needed 8 DEGREES of loft on his putter to continue and putt the way he was. Now that is a lot of loft to look at down there. So he changed his "hit" instead.