Regarding the conversation with Yngwar, I can confirm that 9 INT is sufficient to exhaust his dialogue and get the sketch of the Sormibaeren Champion's grave. You have to be very careful when talking with him, however. He is very easily perturbed. Definitely save beforehand. It may also be possible with 8 INT, but I can't confirm that. 9 INT works, at least.
Regarding the Philosophy III perk and the dialogue with Ferryman, I can confirm that 9 INT is sufficient to exhaust all of his dialogue and get the Philosophy III feat, as well. The problem with getting this feat is that you have to return to
all of the various dialogue options multiple times (at least once for each of the options), because suddenly a new dialogue option will be revealed for each one after you progress sufficiently through the dialogue. It's annoying and arbitrary and I don't like it, but that's the way it goes. Be prepared to spam through the "Tell me about XYZ again" dialogue until something new pops up.
You'll know when it's over once you have the Philosophy III feat (exit out of the dialogue to check) and he asks your overall opinion on his wacky philosophy. Give your opinion and he'll give a very short and uninteresting response depending on what you chose, and that's it. Take the feat and leave, and never think about it again!