I just assumed the relations naturally went down to very poor. Otherwise I have no idea how I could have upset someone on the other side of the world who I had no diplomatic relations with in the first place. I find that if I don't try to curry favour, every nation holds me in poor/very poor regard.
That's because you're number 1. Everyone progressively hates the first, second and third best factions, and everyone progressively loves the last 5. Pretty normal in my book. No one likes the US, but the Caiman Islands are groovy .

That depends from how deep is the gap to fill (sometimes, it's hopeless); in my games, I try to imagine what I'd do if I was a 2-provinces-left faction bordering with an empire with 30.

The first priority is not to make him smaller, the first priority is "survive; make the things the way he'll never attack us until we have rebuilt a decent kigdom by foraging on other factions, no matter if you have to become his vassal, no matter what you pay; we must be on his side or remain neutral, or it's game over".
Yes, but that's what I said : if you're going to lose anyway, might as well try and take the leader down a peg to give others a better chance.
This often happens in board games too btw. During the last few turns, underdog players often try to ruin other players' positions or do silly stuff out of spite/bitterness/fun/to keep things suspensefull to the end, which is why I often play to be second during most of the game. And often win in the end because of it.

But I was wrong before : I rechecked and raids are launched when a faction is outproduced BUT has immediate and local military superiority, otherwise it's invade_none. So they'll raid when they have the opportunity to do so in the short run, otherwise they won't.
Thing is, I don't know how "frontline strength" is calculated at sea... Maybe keeping a strong navy could prevent the blockades ? Maybe what we see as one big sea is divided in zones extending from province coasts à la territorial waters ? I've got nothing.

I don't think that random port blockades should be eradicated: when I first saw them I thought they were intended to represent unofficial privateering actions by hot-headed commanders - the sort of thing that did happen in this period. They need to be toned down and reduced in number, however. And factions should be readier to make redress for them and less inclined to let these actions lead inevitably to all-out war.
Agreed wholeheartedly, but I can't think of a way to do it - as a modder, I can modify and tweak in what circumstances the AI will prepare_invasion, invade_immediate, invade_opportunistic, raid etc..., but not *what* they do when told to do so. The meaning of those different behaviours is hardcoded AFAIK. Nor can I tell AIs how better to react to "not really a real war at all" - it's all War or Peace to them (Tolstoy had it wrong I guess ).