The 'Agreeing Adverb' (Tout)

• Before a vowel: The latent [t] creates a liaison. Il est tout ému. (He is quite moved.) Pronounced [i.lɛ.tu.te.my], the t links smoothly.
• Before a consonant: The latent [t] remains silent. Il est tout content. (He is very happy.) Pronounced [i.lɛ.tu.kɔ̃.tɑ̃], there's no awkwardness. The form tout suffices in all masculine scenarios.
• Ils sont tout surpris. (They are completely surprised.)
• Le projet est tout nouveau. (The project is brand new.)

• If the adjective starts with a vowel sound (a, e, i, o, u, y, or h muet): tout remains invariable. The final [t] of tout links naturally with the following vowel sound in a liaison. Agreeing to toute would be redundant and clumsy, adding a heavy [ə] sound ([tut.ə]) where a light [t] is all that's needed.
• Elle est tout angoissée. (She is entirely distressed.) -> Liaison is seamless: [tu.tɑ̃.gwa.se].
• Elles sont tout heureuses. (They are completely happy.) -> The h muet acts like a vowel, allowing liaison: [tu.tœ.røz].

• If the adjective starts with a consonant sound (any consonant or h aspiré): tout must agree in gender and number (toute or toutes). The h aspiré acts as a phonetic wall, blocking liaison just like a consonant. Without a following vowel, the [t] of tout is silent, creating a phonetic stop. To fix this, the -e is added, which is pronounced as a schwa [ə]. This [tə] sequence reactivates the 't' and bridges the gap to the next consonant.
• Elle est toute seule. (She is all alone.) -> Pronounced [tu.tə.sœl]. The [ə] connects the [t] and [s]. tout seule [tu.sœl] would sound incomplete to a native ear.
• Elle est toute honteuse. (She is completely ashamed.) -> The h aspiré in honteuse blocks liaison, forcing agreement: [tu.tə.ɔ̃.tøz].

Correct. With a masculine singular or plural adjective, tout is always invariable: Ils sont tout contents, Il est tout seul. The phonetic logic that drives agreement in the feminine simply doesn't apply.

It applies perfectly. A past participle used with être or functioning adjectivally behaves just like any other adjective. For example: Elle a été toute déçue par la nouvelle (since déçue starts with a consonant), but Elle a été tout attristée par le résultat (since attristée starts with a vowel).

It is very much alive and essential in all registers, from formal writing to casual speech. Because its function is euphonic (it makes the language sound 'right'), it's not a rule speakers are inclined to drop. Saying tout seule instead of toute seule sounds distinctly foreign or incorrect to a native ear.

You can, and you will always be understood. However, at the C2 level, the objective is to move beyond mere safety and towards native-like elegance and precision. You lose stylistic nuance and the fluid rhythm that tout provides. Mastering this adverb is a key step in that direction.

An excellent C2-level question. In this case, tout is modifying the following adverb (aussi), not the adjective (belle). Adverbs that modify other adverbs are always invariable. Therefore, it's always tout aussi belle, never toute aussi belle. The same applies to tout aussi intelligents, tout autrement difficile, etc.

This is a classic C2 nuance. Historically, the family of héros has a split: héros, héroïne, and héroïsme have an h aspiré, while héroïque technically has an h muet. Therefore, traditional grammar dictates l'héroïsme but l'acte héroïque. However, by analogy, there's a very strong modern tendency to treat the 'h' in héroïque as aspiré. As such, while une femme tout héroïque is technically correct by old standards, you will very frequently hear and see une femme toute héroïque, which aligns the word with its family's phonetics. Both are now largely acceptable, but toute héroïque often sounds more natural to contemporary speakers.