Well, either one or both of your walls aren't straight. That is, they're not parallel.
So, this is really a two part problem:
1. What to do at the back of the counter top which is 1/2 inch wider than the space between the walls.
2. What to do at the front of the counter top which is 1/4 inch narrower than the space between the walls.
So far as the back of the top goes, then I don't think there's any good option other than cutting the counter top to fit between the walls.
If the cabinet below the top is still in place, set some drywall on top of it to simulate the top and use that surface to make a template out of poster cardboard to give you the exact shape of the space between the walls. Mark the top, bottom, front and back of that template so that if you turn the top over you can also turn the template over to ensure that corners line up and that you don't get mixed up and mark the cut lines on the counter top wrong.
"Cultured Marble" sounds like a fancy material, but it's really just Plexiglas mixed with clay or stone dust to give it opacity and a "natural stone" look (kinda). So, cutting cultured marble is really just cutting through Plexiglas, albeit more abrasive, but any circular saw will do it. But, try to get a blade with at least 40 tungsten carbide teeth for a smooth cut.
When I cut plastic laminate counter tops (and I've cut 19 of them so far) I do it with both a circular saw and a hand saw. I turn the counter top upside down so that the backsplash is pointing downward and rest it on some empty 5 gallon plastic pails (all of which are identical in height). I clamp a straight edge (I use a piece of clear fir 2X4 lumber) to the counter top so that the shoe of the saw will slide along the straight edge and the blade will cut along the scrap side of my proposed cut line. (I had this clear fir 2X4 run through a table saw on both sides to remove the rounded corners on it.)
Since a laminate top has a bullnose that sticks down about 5/8 of an inch, I have to be able to lift the saw over that lip without changing the plane of the blade. To accomplish this, I simply make two stacks of three shims each (to make a pair of "ramps") that are parallel to the straight edge to guide the saw over the bottom lip of the bullnose. A line going through the fronts or backs of each ramp would be perpendicular to the straight edge so that both sides of the saw are lifted at the same time by the ramps. I put 2 inch wide masking tape over each "ramp" to hold the shims in place.
My circular saw uses 7 1/4 inch blades and that's enough to cut through the laminate top and the bullnose, but not to cut all the way through the backsplash. So, once I cut through the flat part of the top and the bullnose, I vaccuum up any sawdust on the underside of the top, remove the clamps holding the straight edge down, adjust the position of the part of the counter top I'm cutting off slightly so that the kerf is of uniform width from front to back (thereby ensuring that the part being cut off is in the same location it was before I started cutting) and then set that same straight edge across the keft at the front bullnose of the counter top and use the clamps to clamp the counter tops on both sides of the kerf to the straight edge. That holds the two sides apart a fixed distance and prevents them from moving relative to one another.
Now, I take an ordinary hand saw:
and, holding the blade of the hand saw flat against the good side of the kerf, complete the cut through the backsplash with the hand saw.
And, I see no reason why the same procedure wouldn't work for a Plexiglas counter top.
So far as the front of the top goes, I'm thinking I'd use 2 inch wide painter's masking tape to mask off the wall and edge of the top and caulk the ends of the counter top using a putty knife
to get a nice sharp corner. Wait for the caulk to cure and then pull of the masking tape. Or, at least, I don't think any pro could do any better with this set of circumstances to deal with.