Are airline, phone customer service representatives allowed to tell me about upcoming discounts and offers?

I'm looking specifically to book a one way flight from Milwaukee WI to Phoenix AZ on Feb 28th.

I don't see why they couldnt

Yes

You just have to ask. They may not be able to tell you of upcoming discounts offers, though - only current ones

I don't really see why not. But you aren't likely to find any great deal or discount on a flight that leaves next week. Last-minute tickets are generally the most expensive, unless you luck into a flight that still has a lot of unsold seats (which is not common these days). There won't be any airline-wide deal that will apply to that flight.

They COULD tell you, but it is very unlikely that a regular CSR would even know what any upcoming discounts are very shortly before they start. Things such as this are usually only known to a few "higher-ups" and people in marketing and are not generally shared even internally.

Now, if there's a CURRENT offer that is available they would be more likely to let you know this, but again if you are looking for a flight leaving next week it is unlikely that there would be any discounts. Instead the opposite is possible, at this point the longer you wait the more you are probably going to end up paying.

In addition with some airlines if you book with an actual person over the phone you are charged an additional "ticketing fee" that you may not have to pay if you book it online.

Discount is a term used loosely in aviation as fares fluctuate all the time.
Airlines typically don't have sales and if they do, it's limited to a few seats on certain flights.
Because there are a lot of factors that affect prices, telephone agents won't know when or if a price will go down. Consider that there are literally hundreds of city pairs and each pair could hundreds of possible combinations and each combination is priced separately.
You ever notice when you look at a fare and there are 4 or 5 flights at the lowest price point and the next day there might only be 2 or 3? As certain flights fill up, those flights change prices. Conversely, if flights are empty, they may drop the fares. They won't advertise a drop because fares change too often and there may be thousands of fares that change.

Sorry, for a fligh in less than a week, you are not going to get any discount. You can expect to pay top dollar.

If they have any discounts and offers, they're hardly going to keep them a deep dark secret, are they? OF COURSE they're 'allowed' to tell you: they're ENCOURAGED to tell you.