From what I have experienced, i think a max of 4 or 5 conseutive planets. How i got this, well, i build trade port(s) on all planets, and if you have trade ports in other galaxies, the trade route will follow insterseller space

My line was well beyond 5, probably 10 or 12, and the next planet was rather back upon itself,perhaps around 300 degrees

The trade ships sometimes dont move (same as lots of refinery ships), but that shouldnt make a difference, you should still get the income from them. And i honestly dont know about the length thing then, youll have to ask a dev

Whats the point of killing trade ships? Does this cost the enemy money, or give the attacker money? Or is it stealing the money lost and giving it to the attacker? I see pirates targeting trade ships. I Can't understand it because I don't have to pay to replace them.

You gain money from killing the ship and the owner doesn't make any money from that ship until it is replaced, essentially losing potential income.

Beware Advent trade ships because they have a tech that puts bounty on whomever kills their trade ships.  One or two isn't a big deal, but it can rack up quickly if your are camping a star or other major trade route.

The income a trade port gives is determined by the longest trade route. It doesn't matter if the trade port is on the route or not, they all give the same income (outside of planetary modifiers that boost or reduce trade at that planet). Making the route longer is really strong because it benefits every trade port you have.