More importantly, what is to be thought about my capital, in the center, where I'm seeing a -5F total food supply in the long run? Does this mean that I should eventually plan on farming the flood plains and plain-plains? Alternatively, should I plan on the city pop in my capital never maxing out? Should I abandon that site and move my initial seeded settler to another site (keeping in mind I wouldn't know where with the fog)?
Well, it's unfortunately not the most balanced of situations... settling in-place gives you far less Hills for production purposes, while it provides you with more Grassland squares, so you won't need as much Food. So, yes, settling in-place I would put Cottages on your Flood Plains.
By moving 1W, you enable yourself to work more Hills squares--you don't necessarily have to work ALL of them, but certainly working the Plains Hills River to the N of the Plains Hills Wine is a good one to work, as is the Grassland Hills square to the SW of the Plains Hills Wine--the former because it offers you a free Commerce by being next to a River, while the latter because it only uses up 1 Food to work it.
I would probably Farm most or all of the capital's Flood Plains under such circumstances, because having a capital with access to a LOT of production can be very valuable if you want to spam Wonders there.
Your Fish + Cow City to the south, though, should be moved 1N. As it stands, it will get 4 less Ocean squares and 1 less Coast square by being settled 1N. In exchange, it will pick up a Grassland Hills square (which is pretty good production square to work in general) and it will be able to work a Plains Hills square that the capital will probably not be able to work for a long time. It would also be able to work 2 Grassland squares that could be Cottaged.
Pre-Bureaucracy, the Cow + Fish City could work the Cottages, while after Bureaucracy and once your capital is large enough in size, the capital could take over the duty of working those matured Cottages. Essentially, you'd work on maturing the Cottages when one City is relatively equal to any other City in terms of base Commerce output (pre-Bureaucracy) so that when the capital's Commerce has more value (post-Bureaucracy), it can immediately start working Cottaged squares that are immediately starting as Hamlets or Villages.
Also, I would suggest NOT getting into the habit of using the World Builder for City placement--it can be too easy to get caught up in finding the "perfect" City placement and being tempted to cheat in order to get there. It's better to learn how to deal with the information presented to you at the time, even if the results don't work out to be ideal.
That way, you'll more highly value the exploration of your initial Warrior or Scout on their very first turn and you'll become much more effective in "reading" the situation as to which direction you should send your first Warrior or Scout before deciding where you want to settle your capital.