Here's the deal:
Nobody is going to rip you off. If they don't have what you reserved, you're going to get something better at the original price. My problem is that I don't want better, I want what I ordered. I can't jam a bigger car into a field gate cut-out in Scotland or a small parking place in Tokyo.
Why call somebody else with a problem? Solve it right there yourself. It takes two seconds to walk to the next counter. Business is competitive, the new guy will always match the old guy's deal. If the old guy doesn't have your prepaid money, you've got both freedom and leverage. Total delay is whatever time you spent in the first line and you're off and running.
Everybody seems to agonize over a rental car. It's something you can take care of at the departure gate while you're twiddling your thumbs. If I don't get it done there, I sneak around and do it on the roll. If not then, it can be done waiting for the plane to unload. All that matters is that you get the reservation in the system before you get to the counter. The whole process takes less than five minutes and most of that is entering your personal crap. If you checked the price of a car in Munich for tomorrow way back in December, it's going to be about the same as it is today and you've got exactly the same odds of getting what you reserved.
I probably rent a dozen cars internationally in the course of a year. All I do is stab Kayak, skim for what I want, then compare the unknown of a consolidator against an actual agency. That's a thirty second process.
I've never understood why you need to talk somebody and have your hand held for such a simple process.