But, how far from Brandeis does their protected contour (from pirates) extend? The WBRS signal is already very weak in the areas where the pirate from Boston interferes.
It's the other way around. Any radio station only is protected from interference inside their protected service contour. In WBRS's case, that's a pretty small area; only to
about Chestnut Hill in Newton/Brighton.
However, if the pirate is co-channel then its 40dBu F(50,10)
interfering contour is quite large. For reference, look at that R-L map I linked to. WBRS's 40dBu contour is the blue one and it over most of Boston and then some. So it's quite possible that WBRS is receiving predicted interference within its protected contour. Tough to say whether it's receiving perceived interference. That is, people actually hearing something interfering with WBRS.
But it doesn't matter. Any pirate on 100.1 inside 128 (heck, inside 495) is going to have a 100dBu F(50,10) interfering contour that overlaps the protected service contours for WZLX 100.7 and probably WCRB 99.5, too. Never mind that it's illegal to broadcast without a license unless you conform to the 47 CFR 15.xxx rules in the first place.
Would FCC approve a higher tower with lower ERP?
Yes. But the City of Waltham would almost certainly shoot down any attempt by Brandeis to construct a taller tower. It'd never get past the Zoning Board of Appeals. Nor will the City allow them to move to the water tank (they've tried). WBRS *could* move to one of the Bear Hill towers by Rt.128, or to the WWJD 1150 towers in Lexington, or to WZBC's tower on Chestnut Hill, or even to one of the four Newton/Needham towers. Each would offer substantially improved coverage over population centers, and Class D's are regulated by contour and by TPO, not ERP. So as long as the contours work, you can have effectively unlimited height.
The problem is that all of them, save Newton/Needham, put the campus in the "shadow" of the hill the campus is on. That means on-campus listening becomes very hard and politically that's a non-starter. Theoretically they COULD try to put an on-channel booster on the campus. But it'd almost certainly wreak havoc with reception in the actual city of Waltham, which is a no-no with boosters. And it'd be tough to do more than 3 or 4 watts ERP without exceeding the contours of the primary. At low an ERP, it's not really worth it.
Now, Newton/Needham - because of location and surrounding terrain - is problematic contour-wise with WWFX out of Southbridge. It's do-able, but it requires fairly low height on the tower and a very directional antenna. With that low a height, why spend the insane amount of money you'd need for tower rent on those beasts? Especially when the current tower rent is free?
Finally, ANY move off-campus would make reception of the station on-campus a dicey proposition. Many of those campus buildings are thick concrete or copper-plated exteriors. At 25 watts ERP, they need the proximity to "punch thru".