Sixth grade is where order of operations becomes a whole-expression skill: parentheses, exponents, and knowing that multiplication and division are done left to right rather than multiplication first. These sheets put a squared or cubed term in every expression, which is the piece that is new at this grade.
Every expression is built so that working left to right gives the wrong answer - a student who has the rule can tell them apart, and a student who does not cannot. Drop the level under Expressions for the fifth-grade version without exponents.
Ratios and rates, percentages, dividing fractions, negative numbers, the coordinate plane, and order of operations with exponents. This page covers the last of those; the others are on the roadmap.
Squares and cubes of small numbers only. This is the E in PEMDAS, not a full exponents unit - powers of ten and negative exponents are a separate topic.
Yes - set Expressions to With parentheses for the fifth-grade version, or lower still for two or three operations with no brackets.
By design. An expression that gives the same answer left to right as it does with the rules applied tests nothing. Every problem here is one where the order matters.