I think a better question is... With the Winter Mantle, is Harry now part Fae? We don't know if it bestows longevity on the wielder, but it does give them a faes' weakness to iron/steel - see Harry & Fix getting hit with nails and losing their respective bonuses from Summer & Winter.
I'm unconvinced that they're inherently any more vulnerable to iron/steel than before the Mantles. My read on it was more that the iron blocked the Fae power from doing anything for them, thus dropping them back to where they'd be without the Mantle, which is a sudden shock to the system, especially the longer and more dependent/used the Mantle has been.
I suspect that the situation is more akin to that of Odin/Kringle - While actively wearing the Fae-origin Mantle, they've got Fae weaknesses and limits, but if they were to somehow learn how to not be actively wearing the Mantle while still possessing it, they would not have those weaknesses and limits.
Odin/Kringle is treated as Fae (for most purposes) while he has the Kringle Mantle active, but when he shuts it down to switch over to Odin, he is no longer treated as Fae, but he is always treated as Aesir.
I think an analogy would be ... psychoactive skins (from D&D3.xe, psionics) - you can have more than one on your skin, but only one is active/primary at a time. With mantles, you can have more than one, but only one is active/primary at a time, though with mantles you probably get some passive benefits for mantles that aren't currently active/primary.
As for Harry's longevity ... I doubt we'll ever see it, although Fix's predecessor (?Raoul?) was pretty old but apparently sufficiently fit that he could probably have beaten Slate had he not been bushwhacked. The Fae Knighthood Mantles may well increase longevity, but Harry's already a wizard - and a powerful one - so his natural lifespan was already substantially greater than his probable life expectancy (let's face it, he's never gonna be able to afford life insurance). I suspect that the Knighthood Mantles help keep their bearers physically fit and able longer, even if they don't actively extend lifespan, plus the whole gaining Fae magic may also have an effect similar to that of humans having magic, although that is purely speculative.
Most Knights are generally interchangeable/readily replaced, however, so I'm uncertain as to whether or not the Fae would care to invest the power to actively boost the Knights' lifespans beyond the passive effects inherent in enhanced physical fitness and health plus the magic channeling aspects. Besides, Knights aren't likely to die of old age anyways, and are probably fairly likely to die fairly young while in combat with someone or something.