Engine shudders when tachometer falls to idle RPM from around 2K RPM

As GdD says in his answer a failing MAF is the likely culprit here - and would be consistent with the codes you are getting from the ECU. So that is where I'd start.

If a MAF clean/replacement doesn't resolve the symptoms then another potential cause (based on the symptoms you describe) would be the Electronic Throttle Module (ETM), these were problematic on the T5 engine (especially on early 2000's models) to the extent that Volvo extended the warranty on them if I recall correctly. And the early symptoms of the failure often presented themselves as stuttering/surging when revs dropped to idle.

So far so good with the MAF cleaning, haven't had the codes pop up for about 850mi post cleaning and the MAF housing's interior and the laminar air screen were definitely dirty. I'm assuming the actual sensor itself was likewise dirtied up though I couldn't inspect any real dirt on its parts.

I gave the part a good squirt down with CRC 05110 Mass Air Flow Sensor Cleaner probably using anywhere between 25-30% of the 11oz can.

Here's some of the dirt that rinsed off that I managed to direct onto a fast food paper lap mat which you can see while the dirt was still suspended in the solvent. I follow this with another image after the solvent has evaporated (I did an artificial histogram autobalance to this second image to lend better contrast).

Here's an image of the MAF from the air filter side, you can really see the accumulated dirt on the surfaces. On the inner wall of the MAF housing you see this sandy dust like sediment too

I definitely got the unit much cleaner than when I started. We'll see how long the fix lasts if this was indeed the root cause.

UPDATE I didn't realize it but the dirty MAF really did add hesitation to the engine, the gas pedal is a lot more snappy after this cleaning and I'm pretty sure the mileage has gone up a few mpg. I'll certify that in a future update since I've been doing a lot more highway mileage these days than normal. This hesitation degradation was very much like a frog in the kettle sort of behavior.

UPDATE 2/18/2018 The shudder at idle and DTC P0101 code returned this evening. I last cleaned my MAF at 85,182 mi and tonight I'm looking at 88,096 mi for a total of 2,914 miles driven. I ordered a new MAF sensor to arrive tomorrow to see if that settles the mater. I'll try another cleaning, maybe my air box is buckled or bowed out or otherwise not seated properly if the sensor comes back with a bunch of dirt sprayed off like before.

UPDATE XXX Turns out the root cause was a plugged PCV nipple hose like seen in the black hose for the following part: Volvo PCV Nipple - Genuine Volvo 30731007. I did a full PCV system replacement after measuring an underspec vacuum pressure in the crankcase as measured by my diy manometer as seen in this answer. Here's a youtube video of that nipple hose being replaced with a few closups at the narrow diameter hole that gets clogged.