Yeah, a team made out of Laxus, Erza, Mira, Natsu and Gajeel which would basically be the strongest possible team they could make without Gildarts. But now you're getting into completely alternate scenarios which changes things a lot more.
The problem with Hidden is that it was a game of detection and precision. Gray and Juvia did poorly in part because they had the worse means of detection. With Gray having the additional problems of being focused on by Raven Tail, and counter attacks being rendered impossible by the rules of the game.
Any member of Fairy Tail would face the same issues as Gray did. Erza is precise, but she has limited means of detection. Natsu and Gajeel have a better potential means of detection, but tend to lack precision. Mirajane has no notable precision, detection or notable lack of either. Laxus can likely be precise and potential for the same detection as Natsu and Gajeel. Really it would probably come down to "Can they handle Narpudding better than Gray did?" which is by no means guaranteed.
Yeah any of them could defeat Flare assuming that Obra didn't intervene in time. But with one team they aren't guaranteed to fight Flare, and could face Jura instead, which is pretty much a guaranteed loss.
Yes, Chariot would have been much easier to handle by someone other than a DS. But that holds true regardless of any scenario. The whole reason Natsu was there in the first place was that he insisted on going despite the name. Lack of power up wouldn't change Natsu's insistence on going. Him participating the first day might, but that would also likely result in Gajeel insisting on going to compete with Natsu.
Yes, they'd get through Bacchus or Jenny fairly easily, and Pandemonium would pretty much remain the same. Like I said, while Laxus might be able to solo Raven Tail still, he'd have a much more difficult time of it. Though yeah, any of them could defeat Chelia likely.
Naval Battle they'd be in bad shape for. Juvia and Lucy were their two best "in the water" people. They placed so highly because Juvia was able to through all the non-Fairy Tail members other than Minerva out at once, then after Minerva teleported her out, Lucy was able to endure being beaten, and Minerva got too caught up in her sadism to care about points. Erza with the Sea Emperor armor should still be able to do fairly well... but once it gets down to her and Minerva, Minerva will likely quickly realize that Erza isn't easy prey and just teleport Erza out during the period where it would mean zero points.
And of course they'd lose the tag battle. So, after day four their score would look something like:
Hidden: 0-10 points
Battle: 0 or 10 points
Chariot: 0-1 points
Battle: 10 points
Pandemonium: 10 points
Battle: 10 points
Naval Battle: 0 points
Tag Battle: 0 points
So between 30 and 51 points. I highly doubt the others will do much better on Hidden. It's just a bad game for Fairy Tail who is much more used to tanking and throwing out indiscriminate attacks. Even if they get as high as 5th place, they only match their original end of day 4 score of 45 points. If they some how excel at that, but land Jura instead of Flare, they end up with 41 points. If they do as badly as Gray, and get Jura, they'd be at 31 points, just above Blue Pegasus.
As for the last day, Gray didn't solo the Trimens. Gajeel defeated two of them, then Gray defeated Hibiki (who is the weakest in terms of combat) as he fled Gajeel. Yes Erza is superior to either Minerva or Kagura. They problem is that she'd still have to take on both, and the injuries sustained when defeating one would make it difficult to impossible to defeat the other. Mirajane could possibly take one of them instead as she wouldn't get tied up with Chelia like Juvia did. However this would be the Mirajane that was reluctated to use her base Satan Soul twice in a few hours due to lacking the magic to fuel it. Chelia is about even with powered up Juvia. Mirajane would still likely defeat her, but stand a good chance of being rendered easy pickings for someone like Kagura or Minerva. Defeating Rufus would depend on who fought him. Powered up Gray defeated him with moderate difficulty, so he'd probably be a pretty even match for underpowered up Natsu and Gajeel. The others would have an easier time.
Likewise Leon would be above Natsu and Gajeel. Natsu and Gajeel would also still be weaker than Sting and Rogue, though it'd be up for debate if DF would give them enough of a boost to take Laxus, Erza, or Mirajane. {owered up Laxus was apparently stalemated with Orga thanks to the magic type advantage Orga had (much the same way Hoteye stalemated Jura). Weaken Laxus and Orga will be much more likely to win. Even if Laxus wins, he won't be in shape to fight anyone else, much less Jura.
Yeah if we swap Gildarts in for Natsu or Gajeel it would give them someone that countered Jura with probably the same results as the current Laxus vs Jura.
In fact, let's go with your line up of Gildarts, Laxus, Erza, Mira and Natsu. Not too much changes before the last day. Gildarts would be a horrible choice for Hidden. Probably demolish a building destroying scores of clones with his first attack. He would give them someone who could draw or win against Jura improving their odds of getting points there. Nothing else changes until day 4 when we'd probably get Laxus and Natsu vs Rogue and Sting to keep a four DS battle. That would raise their odds of winning, or at least getting a draw, but personally I would not bet on Laxus and Natsu pre-power up vs DF Rogue and Sting. It certainly would be unlikely to be a 2v1 stomp in Fairy Tail's favor. All told not much changes other than improving their odds of staying even with their old score a little.
Which brings us to the last day. Assume Gildarts swats Rufus to start with. That still leaves us with likely scenarios where Gildarts defeats Jura but is in no shape to deal with others after. Laxus either loses to Orga, or defeats him but is in no shape to defeat anyone else of consequence. Erza deals with Minerva (Kagura not seeking Erza out due to Jellal probably not being revealed) but is left in no shape to deal with Kagura. Mirajane defeats Chelia, but is not in shape to take on anyone of consequence. Natsu loses to Leon, Sting, or Rogue.
The bottom line is that while it's still possible for them to do well without a power increase, the number of scenarios in which they get wiped out rise. They can probably manage 4th or 5th place easily enough, but first would be a long shot, especially as Lamia Scale and Sabertooth's points would increase from the lack of a two Fairy Tail teams, and a likely change in the results of the tag battle. That's a far cry from Fairy Tail winning undefeated, and more a testament to the strength of Fairy Tail's four S-Class wizards, three of which it was already stated or shown would still be competitive post skip.
