I asked for right of passage in the peace talks, so I need to wait twenty turns before I can attack again. By that time WW would be gone, like VMXA said. I could cancel the agreement, but then I'd be negated, yes?. The other civ's won't allow ROP again for an extended period of time - twenty turns?. ROP keeps the civ's happy with me (unless I abuse it).
The ROP will allow me time to produce more defensive units and settlers. The military police on my main island are obsolete and I need more of them so other civs don't take advantage of that.
I agree about mutual protection pacts, their risky. If you know a certain civ is about to attack it could prevent the situation from happening. You need to be careful though.
ROP doesn't make a difference in terms of number of turns. If you sign a peace treaty after war, it always lasts 20 turns, regardless of what else is involved in the deal. Still, you could use the ROP while you're preparing for war again to cruise some fast units past the enemy's cities and see what kind of defensive unit they have.
I don't know about upgrading your home defenses. A stronger military (as a whole) is a deterrent to other civs attacking you; but I don't think they really care about whether your core is well-defended or not, beyond the extreme case where a city has no garrison at all (they love going for that). I'd be inclined to build one good defensive unit per home city, send the obsolete ones over to America to act as cheap defence, and increase happiness in other ways so that your core doesn't need more than 1 unit/city to keep the peace.
Forbidden Planet (there I go, Forgotten Palace, Forsaken Pillbox, Foreshortened Peanut) placement is a black art, much discussed - do a forum search with this phrase (I mean, Forbidden Palace, not the variants...

My way of placing the FP is in a city that I can see will become very productive given the chance, with other potentially good cities close to it, and with a high level of corruption/waste. This is almost always a captured city, which means I build my FP late; so it costs loads, and I have to either put up with a long wait or hope I get enough MGLs to make a couple of Armies and rush the FP.