This yo-ji has always had a certain amount of nostalgic value for me, because its the first 四字熟語 that I learned as such. At the time I was thoroughly confused, not the least of which was due to the fact that 主客 can also be read as しゅきゃく, but also because we learned it as the subject of part of a larger argument about why Japan was going down the drain by some sort of disgruntled commentator or another. You could write a book (and many people have) about the genre of writing about Japan's downfall and its place in the larger discourse about Japanese-ness, but it would probably have almost nothing to do with yoji-jukugo, so I won't waste any more of your time on the subject.
This yoji has a variety of uses, all of which fall under the general category of having the wrong priorities. If you look at the characters, you have 1) a host, 2) a guest, 3) turning, and 4) falling over.
1. Putting the cart before the horse
2. Taking the means as the end
3. Getting your priorities mixed up
The waiters here are always grouchy, so you have to constantly watch how you treat them, even though you're supposed to be the customer. This is a serious case of mixed-up priorities.
So now you may be wondering where the second yoji comes in. Well, let's start with the reading.
I can't profess to have a 100% accurate understanding of the difference between 本末転倒 and 主客転倒, but then again, it seems to me that most people don't, either. Though most people I've talked to about it use the two in essentially interchangeable ways, apparently there is a slight difference in the nuances. 主客 has more of a connotation of "host-client;" you will often see it used to complain about the laziness and corruption of government employees, who are supposed to be "public servants," but instead treat citizens as their personal cash cows. The difference with 本末 is that you have two concepts, one of which is supposed to be complementary or even just sort of ornamental to the other. The thing is, for whatever reason, that ornamental thing has become more important. It has less of a connotation of social norms, I think, than of straight out priorities.
The reason we made the company baseball team was to give everyone an outlet for their stress, but the people on the team have gotten so into it that they're actually putting more stress on their coworkers. With priorities this out of whack, there's no point in having a company team.
Yes, I know, my priorities are out of whack, and I should be studying. But I'm not, so too bad.