Below is a timeline of what you should do if you were injured at work: [...]
This is a subjunctive sentence, but this "should" is not subjunctive. It is like "You should do it now!"
Is this imbalance possible?
"Should" is most commonly used to make recommendations or give advice. It can also be used to express obligation as well as expectation. Source
The sentence you have quoted is a very common use of "should."
"Should" is the past tense of the word "shall." "Shall" used to have a sense of obligation Source, but this sense is retained in "should" but lost in "shall."
Because "should" is originally past tense, I read it as past subjunctive of "shall," agreeing with "were" in the contrary-to-fact conditional.
As you suspected, the tense progression in that sentence is not grammatically correct. Possible corrections include:
Below is a timeline of what you should do if you are injured at work:
This is the most natural and straightforward formulation. If A happens, then you should do B.
Below is a timeline of what you should have done if you were injured at work:
This is what you might write in a retrospective analysis of what went wrong in a response to a workplace accident. (Writing such a report addressed in the second person would be extremely confrontational, though!)
Below is a timeline of what you should do if you were to be injured at work:
This is what the author of the ungrammatical sentence was probably meaning to say. Were to + verb is a special expression that means "in the unlikely event that …". Basically, it's a more hypothetical form of if. Expressing the sentence this way avoids implying that the reader is likely to be injured at work, and more "polite" than (1). Unfortunately, the author screwed it up.