What will happen if I make a parisian flan with all 1% milk instead of whole milk and heavy cream

I have the same preference. Three things that work for me:

(1) If you are using any vegetables, saute them to get as much liquid out as possible in advance. For spinach or other greens, actually squeeze the water out using a tea towel or potato ricer.

(2) Use less sauce or make the sauce thicker. You can accomplish the latter by starting with thicker ingredients (tomato puree vs. chopped tomatoes), or by reducing it on the stovetop somewhat first.

(3) For ricotta fillings, indeed add a couple of eggs to help them setup.

Here is my spinach and ricotta lasagna recipe, which as you can see from the picture, sets up very firm and sliceable. And I've also got a summer squash and portabello mushroom lasagna which illustrates points 1 and 2.

I would likely work the cream cheese first, then try adding the liquids from most to least viscous (condensed milk, evaporated milk, eggs, vanilla)

If you try to use a mixer with cream cheese and lots of liguid, it's harder to combine -- you end up getting floating bits of cheese in the liquid (likely what you mentioned as a 'curdled mess'), rather than it fully encorporating. Having the cream cheese at room temp helps, but is still problematic.

It's possible that you might also want to strain the mixture when you're done, as the mixer might leave the eggs' chalaza intact. (if it's too thick to easily strain when done, you can either whip the eggs first and strain them before adding them, or just skip this step)