So I tried using Gwaihir last week at 500pts and it was pretty well once I got over the urge to cackle as morgul Knights were hurled about, so I'm not rying to fit the windlord into my bigger lists. Here is the first try
Elendil on horse with shield
12 Warriors of Numenor:4 with spear and sheilds, 8 with shields

Captain of Numenor with shield
12 Warriors of Numenor: 4 with spear and shields, 8 with sheilds

High Elf Storm caller
12 High elves:4 with elven bows, 6 with shields and spears, 2 with elven blades

Gwaihir
40 models, 6 Might, 8 will, 4 fate
750pts
I've been thinking about maybe swapping the high elves out for lindr on horse in heavy armour with 5 knights with shields and giving the captain of Numenor a horse, as it gives me tough warband, more bows, and speed, though the storm caller gives me some magic, especially strengthen will which could be very usefull to help Gwaihir out.

I don't want to critique your list too much but I wouldn't take Gwahir in this army list. The main problem I'm seeming is that all your models are expensive and you're making your list quite fragile because of it.

Elendil is great but you pay for it in points, at 750 I would only take 1 big hero and Gwahir is also quite expensive. Maybe switch Elendil out for Isildur he less powerful but with the new ring rules a shield and a horse he's no slouch either

I would swap out the High Elven Stormcaller for Arwen if you have her she is worse combat wise but Nature's wrath is so much more useful than strengthen will and you can put her on a horse so you can get nature's wrath where you need it.

The Lindir/knights idea isn't bad I actually really like it. You'd have a really maneuverable list I'd imagine you'd split your knights between Lindir and Gwahir once the game starts and have them skirt around your main block of infantry hurling troops around and taking pot shots with the knights before charging. However your army would be really small (like tiny) so would require some practice to use well, taking Isildur may help there it would free up points for three knights

The main issue with Isildur is that he is only fight 6, yes the ring isgreat but if he gets charged by any monster bar an isenguard troll when he doesn't have it he is dead because of rend and most monsters being fight 7 or more, least with Elendil he has the will to not be shut down by a single wraith/barrow wright.
The problem I have had with rivendell knights is that some games they are brilliant, but others they just fail horribly, again usually due to the likes of throw and sorcerous blast dismounting them quickly, and they do cut into numbers a lot, so I'm not sure I would want to take more then 5.
Awren is both unfluffy due to the Numenorians, but also the times I have tried her, she just failed, natures wrath is nice, but 3 will and 4+ to cast mean she will at best likely get it off once. Plus I'm thinking if I went for the elven infantry, the storm caller could be good for boosting the other hero's will or blowing monsters away from combat.
I'm going to be using Isildur in my 500pts list, so will see how he does with his one will, and maybe try him out at this level.
If I were to drop Elendil for Isildur, it would be tempting to also drop Gwaihir and try for a horde list by swapping them for rivendell knights, led by Lindr, getting me 45 models, though no really powerful heroes.