I have never even been to Falkreath on this playthrough, and I still got an invite from the Jarl. I am certainly not thane - you do not need to be thane to buy property. My last character that I scrapped had Lakeview MAnor before he was thane of Falkreath.
Well why are you worrying about not getting a letter? They mean nothing, they are just hints to give the player some idea of what to do, you do not need a letter to go speak to someone. You were in Morthal, why did you not just go speak to the Jarl and her steward to see if you could help with anything. You still have to do some of the quests to become thane, they are not going to let you buy land without working for it.