I don't get it. How's that writing is not an issue, yet dragonchilds suggestion of reading it once isn't possible because the config file will be updated. What is it?

In fact, the more I read, the more I get confused about your priorities. Reading in the config file needs to be efficient, yet the config file won't be updated often. It needs to be reliable and efficient, but a database can't be used. You're looking for alternatives, but it's unlikely your infrastructure will be changed. About any suggestion that's being offered is shot down because of "efficiency" or "reliability" reasons, or because of company politics. That doesn't give much leeway, does it?