In addition to being a buffer, early production and research needs to directed to key wonders not killing Freddie given you can’t keep his land.
I wouldn’t put my spy points on him if you’re going to take his worker. He’s likely to put his points on you if you declare war on him and I guess there’s a risk that he won’t open borders. My understanding is that both of these factors would increase the cost of stealing techs from him. Lizzie seems a better bet to me.
I thought about this. All your arguments are valid.
The problem is that I want to steal technology from him earlier - before i'll get Galley and even learn Sailing.
Also, as I see in XML file Frederick agrees to open the borders and trade techs even he is annoyed. The same is for happy resources and only for health resources he needs to be cautious. So, until I get -9 with him it is fine. About espionage weight - you're right, he will give me higher weight. But the same we can say about Elisabeth too - right now she knows only me, it will be hard for her to meet anybody else, as she is on the island, so she just don't have another option.
May be the best person for espionage will be none of them, for example, Caesar or Isabella ( that I currently haven't met). Will try to think on it each move analysing the new information.