In order to marry an NPC, he must be in the proper faction. To add him, enable the console, click on Elrindir and type:
addfac 19809 1
Then:
OK, good news! After further investigation, I've found out that Elrindir should have marriage dialogue: his voice actor also voiced Odfel, who is a legitimate candidate for marriage.
Yeah, try that. Also make sure you're wearing the Amulet of Mara--and that you've talked to Maramal in Riften previously so he could tell you how marriage works in Skyrim (you can't marry anyone without having done that first).