Jull, here is a fairly good article on doggie anal glands.
I found it interesting.
You might want to check out YouTube video's on it. I would apply a warm compress or a warm hot-waterbottle or pad for twenty minutes or so before doing the job with rubber gloves. The one vet had the video set up so badly you couldn't see a darn thing. He wore gloves, and greased up before going into the anus at the four and eight o'clock position to do the squeeze. My vet used a mountain of cotton batting but I was never able to get it to work. I liked the article's better & thought it had some good ideas. (Will I ever be able to look at a clock the same again.
Re garlic and onions. I used to eat them and thought they were healthy. However there are a number of interesting sites that claim otherwise. One suggested that WWI soldiers would rub garlic on their bullets so if one just winged an enemy he would die from the poison in garlic. What ever the truth is I gave up garlic though occasionally cheat. But I definitely wouldn't feed it to my pet.
Good luck. I like the warming idea also.