Well, Natsu also says "Laxus lends me strength" whenever he used LFD, but I don't assume that he's literally tapping into Laxus' mp reserves. If you believe that Natsu can draw on some random power just by being a member of Fairy Tail guild, fine. I just don't believe he can draw upon the mp of other people.Wow a lot of things happened here ... Anyway
Well , Natsu said he's using the power of the guild , I believe him. Because we know now that such statements in FT are to be taken literally if the series has to make sense. The guild mark glowing suggests the FT guild itself granted him power , hence the "guild's power". It's not just emotions , he referenced the power of the guild as well.
Please dude , the 7 Dragon Slayers fight is already bullshit as it is , and I'm trying to make sense of this fight. If we assume it was FT which boosted his power and gave the guild's boosted power to him , at least it would make some sense that Guild Mark Natsu with PoF > DF Natsu and Igneel Natsu. I just find it hard to accept the notion that PoF > DF and Igneel Natsu.
The Zeref and Acnologia fight were different, and Natsu didn't say he was using the "power of the guild" in his fight with Acnologia. Technically you could argue that the guild's power was used for Fairy Sphere, but actually the guild's power was not enough. The whole continent's power had to be used to cast a fairy sphere strong enough to actually trap Acnologia. If there was any sort of hidden power-up in that fight, it would be the boost he got from hearing Lucy's voice. Wendy explained that the voices the dragon slayers were hearing were giving them a power-up. That's why they were able to break out from the crystal pillars.