I am having trouble with my paint breaking up on the methylcel. I am using the extender in order to create clear spaces for my marbling, and I am having so much trouble. Could you give me directions on how you create spaces for marbling?
If you are adding the extender after a lot of paint has already been applied, the uneven edge can become more pronounced. Extender doesn't have any pigment, and therefore less surfactant (pigments are dispersed in surfactant before milled).

Instead of thinning just with water, try adding Acrylic Flow Release to the water first (10 parts water to 1 part AFR) and test it on a bath with a fair amount of color. We would suggest doing a feather pattern and not on a stone pattern for testing.

Make a couple of dilutions such as 1:1, 2:1, 1:2, and apply in very small dots. Sometimes a little white added can help even it out as well.

When marbling is there a Methylcel, which is better than the other? I was wondering if this might play a role in my problems with the extender. When I drop it onto the paint in order to create holes, it also makes the paint fuzzy. I am diluting with distilled water, but continue to have problems, no matter what the dilution is.
One of the things about methylcellulose that doesn't get talked about much is there are many grades of it. It's made by Dow Chemical and there are food grades, industrial grades, and others. To make matters even more confusing, within these grades, there are ones meant for use to achieve specific viscosities. We use the kind Pro Chem and Dye sells because it seems to work well. Your troubles may be from over thinning or adding too much powder to the water. We measure by grams, not volume and we do not have many problems. We also don't add vinegar to neutralize the medium, but we are not overly sensitive to ammonia.