When I said Natsu could use the Demolition Fist in a barrage of attacks, I was more referring to Natsu having previously strung together a lot of attacks fluently and proposed he'd be able to the same, but with fewer attacks at the same consistent power level when it came to using Flame Dragon King techniques one after the other. While the Demolition Fist is certainly a stronger attack, Natsu wasn't too vexed when it came to using it on the War God, mind you when he attacked Jacob with it he did it in FDKM and was still recovering from the tumour nearly killing him, so being hindered by that could possibly be why he was tired after the Demolition Fist on Jacob, but it mightn't be the case as well.I genuinely admire your 10 pages of persistence on this battle and your arguments, haha.
I (personally) equate a move like Natsu's Demolition Fist to other top tier power-type moves such as Jellal and his grand chariot. After re-reading the above mentioned fights (Natsu vs. Jacob, Natsu vs. God of War) in which Natsu used Demolition Fist, I haven't seen any "barrage" of Demolition Fist -- only one really powerful attack.
So for moves like Natsu's Demolition Fist with Jellal's Grand Chariot in which there is consists a large amount of power behind each attack, I don't believe it's possible to use a move like that in rapid succession. For example, after casting Grand Chariot, Jellal was noticeably tired. After using Demolition Fist against Jacob, Natsu was heavily breathing. So yes, I agree with you in that if Natsu "used powerful attacks like the Demolition First in quick sucession in a barrage on Serena" Serena would definitely not win. However, I don't feel it correct to assume that Natsu can do this because it has never been shown that it is possible to fire large attacks like Demolition Fist in rapid succession and retain that amount of power per hit.
As for "Serena feeling very discomforted just from the sheer heat of the mode" that wouldn't apply because since (dictionary definition) Dragon Slayer can eat their own elements, and nothing has been shown to say a person with multiple lacrimas in their bodies can't eat multiple elements or anything related to that, God Serena would probably not be bothered by the heat either. (And I can understand if you would like to disagree with that as it is the backbone of your argument.)
I personally believe this is just a bad match-up for Natsu, even if he's stronger than God Serena in terms of feats and power display.
I would and will disagree with Serena having the ability to eat elements and have resistances to the elements he has lacrima for as per my earlier posts, but even if he had a resistance to fire, I think Serena against FDKM Natsu would be a bit like Natsu vs Zancrow, where Serena would still be feeling the heat from Natsu's attacks like how Natsu felt Zancrow's attacks be 'hot', so he'd be feeling some discomfort still. And there's my other points about Serena's attacks being moot against Natsu as Natsu can evaporate the Sea King Dragon attacks, dispel the Gale Dragon attacks, eat the Purgatory Dragon attacks and avoid the Cavern Dragon attacks, so Serena is still at a loss when it comes to damaging Natsu there, and if it came down to pure force of attacks, from what we've seen Natsu in FDKM, or Dragon Force would still overwhelm Serena and take him down. I mean, Serena hasn't got feats on the level of Natsu's FDK Demolition Fist or FDK Roar when it comes to AoE and attack power, and Dragon Force Natsu would absolutely blitz Serena in terms of speed. So with all of that I see Natsu winning this.