1) Luigi's Mansion: a couple thousand builds ago it was reported that the hose of the Poltergust dissapears during one of the cutscenes, and that it was a CPU timing issue. I'm not sure if the timing was ever fixed, but I can see that the issue isn't happening (check the link, refer to attachments 1 and 2). Please also note that CPU Clock Override is disabled. 

Then there's that other issue where Luigi's model would get corrupted during the intro and Single Core is required as a fix. While disabling Dual Core does indeed fix the issue, there's another workaround. Disabling "EFB Copies to Texture Only" will help reduce the flickering on Luigi's model, and disabling "Defer EFB Copies to RAM" as well will fully get rid of it (attachments 3 and 4). If your PC doesn't have enough power or is too old, like mine, you can enable these back as needed. This only affects the intro cutscene.

2) Now for the second. I seem to have found a new audio issue with Mario Kart Double Dash, a pretty interesting one. On the title screen, scroll down to "Options" and test the sound mode there. If the game boots using a language (say, Spanish as in my case), the "Welcome to Mario Kart!" phrase sounds garbled right when Mario's voice pronounces the "r" sound in "Kart", both on Mono and Stereo mode. It doesn't happen when setting the system to English in the IPL. Using the WASAPI audio backend seems to also fix it.

On the graphics department, the issue of the missing lens flare was confirmed. I will share two screenshots that you may include on the game's wiki page for documentation purposes (attachments 5 and 6, included in the link). Also, I found a solution for fixing the trail effects during the credits sequence. 

If you have pretty good reflexes (and eyes), here's how to test it: load the save state (refer to the attachment) and wait for the credits to roll. Hit Graphics -> Enhacements and keep switching "Disable Copy Filter" on and off, you should see a small change in the angle of the trail effects; please refer to attachments 7 and 8. If your reflexes aren't that quick, try setting the emulation speed to a lower percentage (like 50%). You may use the images for the wiki page as well. Sadly, the characters are still not blurred like they are in console.

I made a FIFO recording of 661 frames to help diagnose the missing character blur in the credit sequence, available at the Drive link below.

And, that ends the report. Yes, I know it ended up being very long, but whatever. I'm open to any questions.

Links:
1) Screenshots: https://imgur.com/a/bVGT0QB
2) FIFO recording: https://drive.google.com/file/d/1BLVNPItw1Cemn29f2Kx9StpkzhxS-0eZ/view?usp=sharing