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maf is only one part of the air to fuel ratio calculations ECU uses to maintain it optimum level and also regulate idle or slowing down rpms.
From your description I gather you have an automatic transmission.
You wrote it stalls when you brake and rpms are reduced.
If you think about it, engine cannot stall when rpms are high. Because it is already turning with enough momentum to overcome its internal frictions. When engine was running at high rpm even if the fuel was cut off completely, it won't stall until its rpms reduce to low level where the rotational energy it has in its moving parts is not enough to overcome the friction and force required to pumping air. Then it will stall.
If you don't give gas and engine rpm is above certain level, engine computer actually cuts of fuel unless you depress the gas pedal. On my car, which has manual transmission, computer doesn't give any fuel to engine while coasting as long as the rpm is above 1K. Once it slows down to 1K it starts sending fuel to it, to prevent it from stalling. I can observe this easily due to that otherwise completely useless mpg gauge they put on the dashboard.
Now as you are braking, engine slows down too. And with automatic transmission when you are in drive and braking, engine has to work against the braking load in addition to its internal friction and other losses. The computer has to adjust the fuel it gives to the engine just enough not to cause the engine drop its rpm too low that would stall it, but not too much that would cause the engine surge and launch the car forward while driver is trying to brake.
The amount of fuel goes to the engine is instructed by computer to the fuel injectors by how long they stay open per cycle. The amount of fuel injected by the injectors also depend on the fuel pressure at their inlet.
So you have injectors and fuel pressure as actors in the correct air fuel ratio.
Then computer needs to know how much air is going into the engine to determine how much fuel it needs to instruct the injectors to inject. For that it uses the MAF sensor. Without MAF sensor it is able to calculate the air amount from engine rpm and intake temperature.
So you have those too.
Then computer needs a validation of what it decided is what is happening, i.e. how much air and fuel gets mixed and burned up in real inside the engine. For that it gets readings from the oxygen sensor on the exhaust.
It compares the values from oxygen sensor to what it was expecting and made adjustments to its calculations on how much fuel engine needs.
So you have the oxygen sensors too.
If there is something wrong with any of those, engine can have trouble maintaining steady idle, which is also related to maintaining enough rpm while you are braking on an automatic transmission car as rpms go down.
As example a vacuum leak into the intake after the MAF sensor causes confusion to the computer, because it is amount of air that goes into the engine it doesn't know about and doesn't use in its calculations (if engine was using the maf sensor)
An exhaust leak close before or even after but close approximate to where the oxygen sensors are can also cause the oxygen sensor report incorrect values. The flanged connection point where the exhaust manifold pipes connect to mid exhaust pipes are common point for exhaust leak with older e90 N52s. The bolts and flanges corrode and start leaking at that connection by time. This is close to at least one of the oxygen sensors after the cat converter. I don't know leak there would cause stalling because it is after the cat converter, so that would only monitor the effectiveness of the cat converter. The precat o2 sensors would monitor how much fuel still in exhaust, those are more likely to cause stalling.
The injectors are very seldom reported to cause issues on N52s. The fuel pump or the fuel pressure regulator maybe. There is a fuel pressure test point at the fuel rail, which is kind of a Schroder valve.
Sensors like crank position sensor, or the eccentric shaft sensor may also cause problem, if they are not giving correct value to computer. Although I would expect you would get codes for those. Not sure though.
Do you see oil on the eccentric shaft sensor connection at the valve cover?
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