You shouldn't be able to make the game unwinnable, even if you messed up those two. As a spoiler, here's how I solved it:
1. quest start
Click here to view the secret text
×Get the letter-delivery quest from Lithrasir.
2. finish that part
Click here to view the secret text
×Deliver the letter to Liff. Get paid. He asks for flowers. Don't tell either of them that the other isn't interested.
3. Haugbui
Click here to view the secret text
×Kill the Haugbui at night. Like, permakill it by digging up its grave, burning the bones and scattering them to the wind. I used sixth sense to find the grave, Flame Aura to burn the bones, put them in the pot I bought from Munavagir, then scattered them out near the ocean. I'm sure there are other solutions that work.
4. reward
Click here to view the secret text
×Got that magic healing liquid from Aurvendal.
5. Use that reward on a different thing
Click here to view the secret text
×AT NIGHT used the cure liquid on that rock formation to the east. Alviss turned from stone back into a svartalf. Do this at night or else he'll turn right back to stone in the sunlight.
6. Finally, the flowers!
Click here to view the secret text
×Eitri was happy that alviss was back and gave me the flowers.
7. Delivery
Click here to view the secret text
×Give the flowers to Liff. He was happy, said "take them to lithrasir" so I did. Lithrasir wants to meet.
8. Bridge troll.
Click here to view the secret text
×Liff can't get to the meeting location because of that bridge troll. Pay the toll on Liff's behalf. I took the cursed ring from Svartalfheim and gave it to the troll.
9. Confirm the appointments.
10. Success! Don't consume the reward you get from this, it's used in another quest (yeah it keeps going).
You should be able to beat the game even if you messed this one up by accident.