I can only use reinforcement magic

Wat

But they are... The only difference is the way they're summoned, but they're literally the same entities

>The only difference is the way they're summoned
That's a massive difference.

How powerful is he? Is he truly as powerful as a servant?

Also, does his Rhos Aias and Excalibur are as strong as the originals? If so, does that mean that UBW is stronger than Gate of Babylon?

OP is already wrong on the first point. Projection is just a more complex version of reinforcement and the VN foreshadows that Shirou can use projection anyway.

>and the VN foreshadows that Shirou can use projection anyway.
They do in the anime as well but it's been a long fucking time since i read the VN so if there's a specific part you're talking about let me know.

They're in different universes because, while at first in the early stages of the Nasuverse things were vague and not developed enough to be able to distinctly say "this cannot happen here and this cannot happen there the in-universe settings don't work like that", Fate came along, got popular, then started getting intense interest in its particular mechanics and concepts from fans/spinoffs. The rest, like KnK, Tsukihime, and Mahoyo, were subsequently neglected conceptually, and the development of the Fate universe started to theoretically encroach on the possibility of them all occurring within the same continuity due to having to rationalize why later entries in Fate and whatnot could occur. To say it simply, power creep made it impossible for them to all coexist reasonably.

There's not a ton of reason for them to make the connection. The mechanics of the other world are completely different in each Persona game, so they had no reason to assume that fucking with Shadows would change the real people (and no way of discovering the Metaverse even if they did guess it).

Rin walks into his storage shed relatively early (5th-8th day in the Fate route I think) and discovers his leftover projections.

Aside from HF where he actually gets archer's arm, he's not as powerful as a servant.

>Also, does his Rhos Aias and Excalibur are as strong as the originals? If so, does that mean that UBW is stronger than Gate of Babylon?
No to all of these. Also, Shirou can't trace excalibur without killing himself.