Substitue for paneer? Have low fat milk, plain youghurt, cream cheese and butter

No, you can't substitute any of those for paneer. This should be pretty obvious given that you're trying to skewer and grill the paneer. It's a firm cheese that doesn't melt, while any combination of milk, yogurt, cream cheese, and butter will melt, if it's not liquid already. The best substitutes would be other cheeses like halloumi that don't melt, or else tofu. A firm cottage cheese would probably also do okay.

You also can't really use a cheese that melts in something like this. First of all, it just could never be the same texture, and the flavor would be a bit different too. But it could be worse than that. You didn't say exactly how much pumpkin, onion, and breadcrumbs you have in there, but generally, if you put a melting cheese in there, it's at best going to make the whole thing less solid and at worst it'll drip out as you grill it or conceivably even let it fall apart.

But you can indeed make paneer with the low fat milk, though. The texture just won't be quite as good - a little tougher, more dry and maybe even crumbly - since there's less fat and relatively more protein. With whole milk, 2L of milk makes ~350 grams of paneer, though that can of course vary depending on how well you drain/press it. Low fat milk won't have quite as high a yield, but you can probably still just start with 2L. The texture is probably the bigger problem. (And it'll of course be worse if your milk is 1% than if it's 2%.)