All I'll say is that this is a double-edged sword. It also drags you down to just making headcanon, as you're also making an unconfirmed theory by piecing together things in YOUR head.
Both of you are using similar evidence types, so both of you would be just arguing over your preferred headcanons by your logic.
The difference is I've only been saying Metal is likely to be an illusion and backing it up with evidence, not saying he 100% must be real and here's evidence being used as if it's proof to debunk him being an illusion. Sure, maybe we are both arguing headcanon, but I was never trying to force anyone to believe anything to begin with. Just backing up my viewpoint with evidence, which I can easily still do.
Having just played that fight, I can tell you multiple things that prove my point. First off, there’s a giant Egg man building that wasn’t present in CD.
There's also an entire section to the fight taking place in a room that wasn't in CD. In what way would this hurt the chances of it being an illusion?
Second off, when Giga shatters you can clearly see regular Metal Sonic fall down out of the illusion. If he was an illusion to begin with, he would have shattered along with his Giga form.
How do you know? It's clear things are visually different in Mania.
Third off, when Eggman is locating Metal in Press Garden, you can see him check on Metal’s vitals to make sure that he’s still running. All of this seems to point that this is a return visit to Stardust Speedway (Little Planet’s time travel functions differently than what we’ve seen in Generations) and that this is the same instance of Metal that we already know
Metal doesn't have vitals. He's a robot. He definitely is displaying information of some type on the screen, but to go so far as to say these are vital signs is a huge logical stretch. And before you say it, it doesn't make sense in the slightest he would have a flicky or anything powering him from the inside. That little critter would have died a long time ago. It's clear that Metal is an ordinary robot (comparatively speaking) and not a badnik.
There's some other oddities I noticed, at the start of Mania, Egg Robos are transformed just being in proximity, but was it Eggman's thoughts or the Egg Robos themselves giving these forms (this might be silly to think cause they're machines though)
Considering robots like Metal Sonic made by Eggman exist, it's not so unreasonable that the Egg Robos could have had a rather complex AI that gives them free thought. It's hard to say for sure though regarding their transformations. We barely even understand how the Chaos Emeralds work in regards to different characters transforming, let alone an entire new maguffin.
Later instead of using the Ruby on Metal Sonic by proximity, he just gives him the Ruby. Why did he need to do this? is Metal Sonic using it as a power supply like the final boss in Forces or is Metal Sonic using it directly himself? Again if it was Eggman it'd be weird he had to toss the Ruby at him when he didn't need to do that before for the Egg Robos.
I'd assume one makes the most out of it by having it near, but doesn't need it to stay near just to transform to begin with. Effectively, it's broken down into two separate things. The first is the transformation itself, the second is whether or not the Ruby is being used like a battery to juice up the transformation once you have it. Eggman was going to considerable lengths to try to make sure Sonic and friends didn't make it out of this fight, that much is clear from him triggering the transformation to begin with. As such, if he was that desperate doesn't it make sense he would want the Ruby to be there making the form stronger to help boost it's odds of winning?
If it was as a power supply I'd find that redundant unless it's the only way to get a more powerful form but Chezi made me realize it had to be directly from the Ruby cause why would he pop out of his form shattering like it was some giant exosuit? Maybe it's both though since he seems to keep it in some dome and fires pink lasers.
As much as I'd love to use the lasers as an evidence of being an illusion, I can't do that. Lasers come in all kinds of colors naturally, and pink/purple happens to be a common color Metal uses in addition to yellow. I've already covered the "popping out" thing as probably just being a visual difference with how Mania handles defeats as opposed to Forces, so this is all fairly inconclusive in either direction. Though, thinking about it, it does kinda boost the illusion theory in that if it is a Metal illusion and it can't actually transform, it would have to wear an illusion of the transformation around itself to mimick said transformation. It's not a very big boost though, because we don't even know for sure if the real Metal Sonic could transform with the Ruby to begin with, so all it suggests is that Giga Metal is probably an illusion.