Go to The Lady Vengeance and take the rowboat to shore, then take it back to the ship. This makes Tarquin appear 99% of the time.

Seems like I am the 1%. He stays at the doors of the graveyard mumbling his "Let me think" dialogue. Sigh.
But thanks for trying to help.

Are you playing with just one character? Otherwise your post doesn't make sense for a couple of reasons. First there is a healthy variety of ways to accumulate your source points--don't think there are "only three" because there are not (I counted 5 or 6 source-point quests that I undertook--but of course you only get three points total--the others give you more XP and a variety of objects, etc.) There may be more--but didn't wait to find out...;) So if you have to then take another quest for your third source point. I also did the ritual three times as you talk to your God every time and he will give you some neat abilities and other things as well after the ritual.

So, in your fight you speak of immunities...you kill one of them with physical damage and one of them with magic damage. Your character(s) should easily be able to do that while having some fairly robust physical and Magic armors, etc. How is it "bad design" if you choose to develop your character's intelligence at the expense of his strength--that's your choice--has nothing to do with the quest design. Especially if you are playing with a single character!

Getting three source points is actually easy *if* you have your characters balanced instead of lopsided...;) The recommended game method is with four players--I have a fighter, a rogue, a ranger and a pure mage. I mow them down now...;) Playing with a single character if that is what you are doing is your decision as opposed to the game's, as the game and the quests were designed for four characters. All my characters have ranged attack skills of some kind, and my fighter does both (he has almost all the warfare skills.)

I think the method mentioned worked in earlier versions when Tarquin moving around had a lot of random bugs, but when patch 559 was released it seems it broke Tarquin for just about everyone in that after giving him the sword pieces he still won't return to the ship. As far as I've been able to tell there's currently no known fix (I've put my game on hold until a couple more patches are released to fix this bug and others).

You have seriously failed at understanding the problem, bro.
1) You get that last mission after you have all three source points, so getting another source point is obviously not the issue.
2) That is a solo mission that you cannot take your teammates on but is only for your own character. How balanced your party is does not matter at all, and I explicitly laid out that my party build is varied.

So please educate yourself before responding with stuff that does not even apply to the situation. Because like this you're kinda just being ridiculous.