Each crow is its own attack, so yes, you can hypothetically have a bunch of lightning strikes, teleports or freezes.
He's alright, though typically a better wall is found with Mr Iron. Scarecrow can only push his luck up +130% (Three lucky pants, two Withered items) through items, and would still fire rather slow. So even with seven attacks going out, he still has pretty low odds at sending a creep/boss back (4.6% X 7 = 32.2%) every (2.8 / 1.4 = 2). Meaning every two seconds, he has about a 1/3 change at sending someone back. Mr Iron, for comparison, can quite easily hit minimum attack speed (>7,500% speed bonus is easy), and boost his luck excessively (>1,000% is not uncommon) and fires two attacks. As a result, he ends up with (20% X 2 = 40%) every (2.5 / 75 = 0.03). Meaning, every two seconds, he will have sent back ~26 creeps.
I generally run Mr. Iron as my main wall, since as you said, his luck and his ASPD can go really high. I'm talking about using Scarecrow as backup wall farther in the back of the map, where none of my DPS towers are placed (mainly because they can't gain level with anything other than keys of wisdom), helping me stay in Bonus Round longer. I also generally run Impatience's Wrath + Withered on my Scarecrows, because it's better than an empty inventory and I usually have very few Lucky pants to spare orz Now that I think about it, I could use more Mr. Iron instead of Scarecrow, since I do get a lot of boots and mushrooms from reshuffling dark forge items. I'll miss out on some cauldrons/barrels/fistful and potions, but it's worth it considering how much more effective Mr. Iron is as a wall, even with his small range.
Yep, unless you're running a deck that prevents his use, Mr. Iron will handily outperform Scarecrow as a support tower. Have fun!