That's the best explanation I've seen so far.

Both camera and filter were left outside for 30 minutes to equilibrate with the outside conditions.

Tonight is clear and I am doing some night sky photography. Its 37 degrees, so have taken the camera and filter outside for 20 minutes to allow temperature equilibration to avoid fogging. Set up the camera, took a half hour sequence of shots, no problem with fogging. Added a screw-on light pollution filter and within 10 minutes, the filter, not the camera lens, is badly fogged up on the surface facing the lens. What do you think the solution might be? The filter is the Hoya Starscape filter. It does a great job of reducing light pollution.

Exactly!

Can you post your comet image?

The key observation here is that the second click does not occur until the Intvl counter has counted down. That means that the exposure did not end until then. So, I did this experiment with my Canon 80D and my Aputure TR1C Intervalometer. Using your setup with camera in manual mode, my equipment behaved exactly like yours did, ie. only every other cycle of the intervalometer produced an exposure. Switching to Bulb mode achieved the expected number of shots. What is going on is that in Manual camera mode, the exposure time is not being controlled by the intervalometer, but is being controlled by the camera. The camera is not looking for a possible signal from the intervalometer until it is all done with its manual sequence. The camera is ready for an intervalometer signal by the time your "interval" timer has expired; that's why you here a click then.
To prove that to myself, I set the "length" parameter on my intervalometer to 40 seconds, leaving the "interval" set to 2 seconds. Sure enough, the camera terminated the exposure on its own, after its timer hit 30 seconds. As soon as data writing to the SD card was complete, it was ready for the next intervalometer command. The intervalometer continued to count down to 0 seconds, then did the "interval" timing and when the next 40 second cycle started, the camera launched its next 30 second exposure.
So, to avoid the camera and the intervalometer "fighting" with each other for control of the exposure, you need to be in "bulb" mode, which you have already concluded. This experiment does clear up some confusion on my part; I'll be sure to use "bulb" mode the next time I'm doing time lapse work.

This may be part of the answer. Let's say that the 100% crop displays a part of the original image that has as many pixels as your monitor does, in other words, 1164 by 776. Now, what portion of the original image, when cropped by 75%, would just fit in your monitor? The answer if 1552 x 1034, very close to your 75% crop values. What part of the original image, after cropping 50%, would just fit in your monitor? Answer is 2328 x 1552.
Unfortunately, this rationale doesn't work for the crops larger than 100%, so I can't explain that. But maybe this is part of the answer, anyway.

Got my SharpStar 2 filter this week, tonight it is clear enough to try it out. Works just as advertised. Easy to determine best focus. Top image is at 18mm, F3.5 while lower image isw at 135mm, F5.6. Canon 80D body.

I looked back at some of our shots from the Kauai helicopter tour and found that nearly all the shots were taken at the wide end of my zoom lens; most were around 18 mm.
You'll love the flight!

(Download)

Use of a card reader is not a two step process. In other words, you don't download your photos to the card reader and then transfer from the card reader to the computer. Instead, you insert your SD card into the card reader; then your computer uses the card reader to transfer images from your SD card to your computer. The card reader will have a slot to insert your SD card into, and a USB plug to insert into your computer. Its very simple, cost effective and reliable.

Don't subscribe, use Corel PSP

We did the Egypt/Jordan/Israel trip in June and enjoyed every minute of it. Took Canon 80D and the Canon 18-135 lens. Occasionally wanted more reach, but not worth the hassle of another lens. We were part of a land tour with Israel Tour Consultants. Never felt threatened anywhere, even in downtown Cairo. Two weeks was long enough only for an introduction. Cairo, pyramids, old churches, traditional Mt. Sinai, Petra, Mt. Nebo, Tel Aviv, 2 days in the Galilee area, 3 days around Jerusalem. Backed up images from SD card every night on WD My Passport Wireless Pro. Shot everything in RAW. Trip of a lifetime.

Thanks jackm1943. I plan to catch both of them, since we'll be at Sterling for 50th college reunion.