The thing about that is, required to oppose each other or no, Winter and Summer are explicitly shown to work together in CD, and every Faerie-themed book does end up with the results of both of their actions benefiting both courts.
If you are referring to Maeve and Lily as I assume you are, that is a very abnormal circumstance. Maeve is infected and Lily is lonely/impressionable and overall too willing to 'do good' for her own good. Summer and Winter are diametrically opposed at all other times, its why every time Mab chose Harry as representative or emissary before he became the Knight, Summer was there to oppose him in some way.
I would argue Summer benefitted one time and one time only, when Harry unknowingly blew up the interior of Arctis Tor with summer fire...something that Mab seemed not all that concerned with anyway...and that allowed Summer forces to come to the Council's aid. Every other time Summer gets shafted, their Lady dies TWICE, their Knight once and the new one gets his ass handed to him. The Gruffs are defeated and felled at every turn, the exception being Eldest who gets sent to be an errand boy...amusing thou he found it. Titania is in a perpetual state of depression now and no longer cares if the world goes down. So i'm not certain where the notion that Summer has come out positively in the end is coming from.
Yes. Because a) Harry's magic fades out before touching it, like with Lord Raith's explicitly Outsider-backed protection, rather than just being bulled through like Grum in SK or Ursiel in DM, and b) its nature is different from other fetches, it looks like the Scarecrow rather than a fetch disguised as a Scarecrow under Harry's glamour-penetrating ointment which is off enough for me.
Raith's protection isn't explicitly stated to be Outsider related, its just assumed at this point and yeah probably likely considering he had the three Strega's calling up HHWB. But for Scarecrow, the reason his nature is different is because he holds the mantle of Eldest, just like Cat Sith, the oldest and most powerful of its kind. Just like the Red King is far stronger and has more capabilities that pretty much all of his progeny. To me Eldest Fetch is just that, and his nature is a bit different thanks to his mantle.
Consider there are plenty of beings that can snuff or shut Harry down with little more than a thought. Ferrovax does so in GP, Odin in Changes, Mother Winter in CD temporarily...these are the eldest, strongest beings of their respective factions. If Eldest Fetch has been around as long as it claimed, AND is a personal servitor to Mab, surely it could possess much more refined magic that doesn't require it to just brute thru it, Ursiel and Grum are tanks but no one is crediting them with any more than that. Scarecrow rubs his status right in Harry's face with contempt, it
knows its power is superior in its own mind anyway, and does not need to bulldoze thru it.