I'm not sure why he seems surprised in the last panel.

Well, I would have translated the last panel:
"Oh! I can see it!
I can see the aura of her feelings!"

The meaning is basically the same as far as I can tell though, I just felt like it had a more natural flow this way.

I think it goes like this:
- he makes a joke that praises her,
- she puts him down for his joke,
- later, while eating, she remembers what he said and she starts to beam happiness,
- he notices it.

I gathered that much, I just figured he would have already expected her reaction at the end, so I wasn't sure why he was surprised.

The last panel is Kyon seeing the dere part of tsundere in action.

is he making a pun on haru and spring?

He's playing with similar-sounding words.

ハルヒー 俺、熱中症
Haruhi... ore, necchuu shou
(Haruhi... I've heat exhaustion)

ハルヒに熱中！
Haruhi ni necchuu!
(Hot for Haruhi!)

ハルヒ熱中症！
Haruhi necchuu shou!
(Haruhi heat stroke!)

Oh Japan, you and your strange, nigh-untranslatable word puns that no one except you get.

I must take this time to appreciate the master linguists who manage to create English puns out of Japanese wordplay.