"Other Bottom-line is: HF antennas should be outdoors."
I agree. I'd go to extremes to get an outdoor invisible antenna up before I settled for an indoor one, especially on 75/80.
I operated from an apartment for a couple of years and had good luck with outdoor magnet wires into a couple of trees near my apartment.
I started with a copper tape stuck on my balcony wall and ceiling (nonconductive) fed against the railing. Much better than that was a random wire into a tree fed against the railing. Another big improvement was seen when I got another magnet wire into another tree and fed the two as a doublet with a tuner on the balcony:http://www.n3ox.net/projects/servo
The signal to noise ratio on the doublet with a good choke balun on the tuner output was much, much better than the signal-to-noise on the wires that used the railing as half of the antenna. In fact, grounding the tuner to the railing increased the noise by 2 s-units on most bands and more on some. I even saw a difference in the noise by moving one of the doublet wires so it was a foot away from the building rather than 2 inches.
The moral is that an apartment building is just hot with RF junk. The less interaction between your antenna and an apartment building, the better.
If it's really, truly impossible to put up outdoor antennas, then indoor is better than nothing.
However, magnet wire is more or less invisible. Find the wire crossing this picture:http://www.n3ox.net/projects/antennas/VHFcontest1_lg.jpg
As you can see from the VHF antennas, I was able to get away with a lot more than one might think at an apartment building. The VHF antennas were temporary; I only put them out when I was using them.
The HF wire was up 24/7. I had to look for it pretty hard to see it... and don't think anyone else saw it.