If you're comfortable with Maple programming language, you can try 'inert form'. It will allow you to grade student response in its original form, without simplifications. Click here for an example.
Alternatively, you can convert the response to "string" and use string tools to check if the expression contains (the right number of) brackets, this should allow you to identify the expression type. I'm thinking alongs the lines of: student answer is equal to the correct answer AND the number of "(" brackets is right.
With regards to "y=" part. It's hard to say without the actual grading code. You could try to pull the the equation apart with use of lhs and rhs commands. For example, "y= a + b" would give you "y" and "a+b" respectively. This should allow you to grade equation in two parts without equation comparisons.
When assigning variable names to the equations (Maple syntax) I often forget to use ":", for example: "my_equation = y = a+b" instead of "my_equation := y = a+b". This leads to similar error messages.