AVR Freaks

Yes

Excessive return current under the MCU will affect its oscillators.

ESD/EFT/lightning suppression is ideally at the edges of the PCBA and away from the MCU; that's not the case on small PCBA.

So, the MCU should have some jitter tolerance for its main clock.

Odd is 20% max is new; IIRC, XMEGA is 10% max and XMEGA have PLL.

 

Really nice project and i am really honestly impressed by your assembly skills. But nowadays, its not really useful anymore in professional work. 
You can do pretty much same thing or much better, even with basic open sourced arduino librarys and a ili93xx supported display, with much higher resolution then 200x240 via VGA and in a fraction of time that i assume it took you to do what you did. My first project when i started with my job, was an arduino with atmega328 running TFT with 480x320 at 45-50fps, via ILI9341 interface(pararell). Took me maybe few hours of codins and was done. While running at 8MHz.

And now i work with STM cortex m1 or m3 MCUs, i mean, those thing run something like 1024x720 TFT display, at 60FPS without any problem and PCU costs like 4 or 5€ even in small quantities.

But i really wonder where you find the will to try to do so much, with so little.

I guess assembly was king of programming back when memory was limited, but now even the biggest project at our company take like... maybe half of memory on a mid tier stm32 MCU. But its still almost necessary to be able to read assembly and understanding what is happening  when debugging cycle after cycle, so you can find out what you did wrong when writing C.

Perhaps with some expensive or really good compilers on the market, assembly debugging is no loner a thing, but i still work with GCC GNU one and its a real handy skill to have.

TBH, if you want to get most out of already existed library, you still have to dig way down to basic functions and clear it up for your specific usage. If you just start throwing in library's all over the place, you can fill even 1MB memory in a flash(pun intended). What i wanted to say that when a customer wants something done, you don't go starting from scratch in assembly and tackle any problem you encounter head on. Nobody is gonna pay for that. You need to get thing working fast and efficiently. I would love to have time to go play with assembly and optimize every single line of execution perfectly, but in real world, you don't have time for that. It is just not worth it in real world, when difference between MCU with 256kb and a 1024kb memory is like 0.05€.  And raising or even overclocking MCU frequency is a biiiiiig nononono. When you try to get a product EMI certified, you should be aiming for lowest frequency possible. 

All in all, its all about final product specifications. If you working on hobby project or something for yourself, then sure, assembly is nice way to take it slow and really enjoy getting into dirt.

But for all of projects i did for my costumers, going with assembly instead of C, would be like if you went to build the house, but instead of just buying bricks and cement, you would buy land with clay and make your bricks from scratch. Making a customer electronics with fullHD display and fluid GUI, a BLDC driver for robotics, medical sensors, long distance communication devices, metering, ... all of that, programming in bare assembly would take EONS.

I never give advice to other people on their problems, unless i am totally familiar with project and system around it. And i never had the need to move int vectoring, so i cant comment on it.

But I would NEVER EVER use a atmega32u4 in a application, where i need to sense something so precisely, that even a 1 or 2 execution's would matter.

Really good example. If your ultimate goal is to get to high altitude fast and efficiently, get on a plane (analogy for using a piece of engineering/code, someone else made and is sharing with you). If you wanna do it all yourself and suffer in progress and take al lthe tiem oyu need, so that final result is more "deservable", then hike to top of mount everest. Maybe even make your own shoes. Good analogy for assembly vs C :P