Genesis 26.5, as you say, suggests that some form of the law existed 700 years before Moses. The terms that are used in Ex. 16.28 are generic, general terms for commands and instruction (not necessarily codified legislation). From the very beginning God had been instructing His people and giving them spiritual and moral direction. That's likely all that is being suggested by Ex. 16.28.

The term for "commands" in Ex. 16.28 is מִצְוֺתַי (mitsvotay), and denotes merely any mandate coming from a recognized authority. The term for "instructions" is וְתוֹרֹתָי (vetorotay), a form of torah, which, though used specifically of the Sinaitic revelation, is also a generic word for instruction, i.e., it doesn't necessarily mean the Mosaic Law. What was given at Sinai to Moses, however, is also called Torah. These commands and instructions referred to in Ex. 16.28 are the generic ones that were followed later by specific Sinaitic laws, and obviously the same term ("instructions") was applied to them.

We do the same thing with our term "law." It can just mean "law" in general, or when used in context (like when you're speeding you're disobeying "THE LAW") it can refer to something specific, viz. our traffic laws.

How do you explain Exodus 16:28 if you believe the law was created with/during Moses?

Those that believe the Law was only during the old covenant, how do you explain Exodus 16:28 of the Israelites having the law before the Covenant? This, alongside with Genesis 26:5, shows that the Law predates the Israelites and should be kept by all who choose to be servants of God. The Law originated and was in force before Moses. It was only codified within the old covenant. So, it shouldn't be called the Law of Moses, but the Law of God. Moses did not create them; God created them and simply gave them to Moses.