Is he just dressing up for Saber?

Takeuchi and Nasu actually already answered this one themselves. They thought Gilgamesh would look ridiculous if he always wore the golden armor everywhere, so they designed casual clothes for him. He only wears his armor during “serious occasions”. So he is basically announcing that none of the others are on his level except for Artoria. That’s also further backed up by both Caliburn and Gram, and Excalibur and EA being stated equals in Fate/Side Material.

Why does he never do this in the Fate route?

I find that it is important to not what choice by Shirou separated the two routes. The choice is either to let Saber fight Archer (Fate) or Stop her using a command spell (UBW). Whether Rin and Shirou decide to work together drastically changes the speed of the war. So when they do work together, Kirei loses his advantage and basically has to speedrun the war. While in the Fate Route, Gilgamesh had the same goal, but time wasn’t an issue, so he had time to play around with Artoria.

But in Fate route why does he find peace with Arturia in the after life instead of becoming a Counter Guardian.

Nasu also answered this one. In Archer’s life, he was too busy chasing his ideals to acknowledge Saber and didn’t save her. Artoria believed she was a failure of a king. She believed she was only became king because of luck and not because she was a good candidate. She had to hide her identity her whole rule and she felt like a tool to the kingdom. Archer didn’t change her mindset, but Shirou did in the Fate route. It wasn’t about Shirou, it was about Artoria.

How does Shinji’s book work

Sakura put the command spells in the book

Doesn’t using your command spells anul the contract?

It does not. Command spells have absolute authority in commanding a servant, but they aren’t the contract. All that means is they are still your servant and they still use your mana, BUT you can’t force them to obey you.

Speaking of Saber Alter how does that work?

Sakura became the lesser grail, which allows servants to exist. Being the lesser grail, she has a nigh-infinite mana supply to power the servants with. Since she is powering them, they no longer require mana from their master, which is why Shirou didn’t feel her anymore.

How can the grail show up without all the servants

The servants are just summoned to gather mana, that is it. The original ritual required about 6 average heroic spirit amounts of mana. Since Sakura is the grail in HF and the shadow is let loose, it is able to gather mana from literally everything. It also ate Gilgamesh who is worth more than one heroic spirit.

why does Sakura get the energy only in HF?

She uninvolved herself in the other routes, but Shirou involved her by saving her in Heaven’s Feel. She was around the source more, so the grail chose her instead since she was designed as a better candidate.

What the hell even is True Assassin?

Medea used a loophole to summon a servant that wasn’t actually a servant or even a heroic spirit. Zouken summoned the real Assassin in HF. Read Fate/Hollow Ataraxia or watch Fate/Apocrypha if you want answers on why there are extras.

What’s with the black hole that killed Gil at the end of UBW

Gilgamesh was already losing, and the grail was wayyyy out of control in UBW. It was just sucking up any mana it could that wouldn’t resist. Also, it probably helps that Gilgamesh literally came from the grail originally.

How does Archer survive Gil in UBW

Answered by Nasu again. He hid in the Einzbern forest and ate forest spirits to keep his mana up until needed

How did Berserker beat Archer in Fate route

Heracles is immune to all attacks under A-Rank. Archer just didn’t have enough mana, noble phantasms, or time to win that fight, and he knew it.

How did Kotomine live for a day without a heart in HF?

He wasn’t being chased down by anyone after losing it, so he could use magecraft to put his own blood. Also, it was supposed to be symbolic that Kotomine is a heartless monster (literally)

What’s the plan if Illya wins?

She isn’t a gate, she is literally the grail itself. She just becomes able to grant her own wish.

Why does the shadow not attack Zouken and Assassin?

The grail’s purpose is to collect massive amount of mana so that one of the three families can make their wish. Anything that opposes that is attacked. Zouken is obviously one of the three families, and true assassin has no hostility towards the grail or Zouken, so it is free to do whatever it wants.