As soon as Alduin starts talking, fast travel to High Hrothgar and then back. It worked for me.
You actual mannaged to get the potion Dragon Research has never worked for me (i know it should be fixed in 1.9).
If you attack Alduin before all the dialog has finished that will cause the invulnerable Alduin bug Paarthurnax will say something like "Now! use Dragonrend" and then you safe to start, like 72.236 said entering and exiting a building can sometimes fix it or like you said go back to save before you use the Elder Scroll (reloading to just after seems often to not fix it).
Yeah it did work for me. I'm doing this again with Lvl 48 Nightblade and the Dragonborn DLC. It should be a piece of cake.
Done was a piece of cake. Deathbrand Armor, Healing Pots and Talvas Fartharon (Neloth's Apprentice). Finished in about five minutes of brute force.
Whoops 47.20.167.171 is me and I forgot to login.
What I did was I went in my Arch-Mage Robes, destruction cost-ring and necklace with the Mage's Circlet and Magicka enhancing boots, I thought I was ready. I had ebonyflesh running and was dual-wielding Icy Spear... and then I realized that I was at stage 4 vampirism and got my face melted off from Alduin's fire. Just make sure you aren't at 50% weakness to fire when you go up against him.