Вам стоит уточнить адрес у администратора перед отправкой документа.

Because the construction кому стоит + infinitive treats the person as an “experiencer” or “the one for whom it is advisable.”
So Вам (dative of вы) means for you / you (should) in the sense of recommendation, not as a direct subject.

Стоит here is not “to cost” or “to stand.” It’s the modal-like meaning it’s worth / you should / it would be a good idea.
Pattern: (Кому) стоит + infinitive

• Вам стоит уточнить… = You should (it’s advisable to) clarify…

This is a soft recommendation (less forceful than a command).

Уточнить (perfective) focuses on completing a single result: get the address clarified/confirmed once.
Уточнять (imperfective) would sound like an ongoing or repeated process (to be clarifying / to clarify in general). In advice right before an action, Russian often prefers perfective for a one-time step.

Адрес is accusative singular (same form as nominative because it’s inanimate masculine). It’s the direct object of уточнить:

• уточнить (что?) адрес = to уточнить (what?) the address

У commonly means at someone’s place / from someone / with someone (as a source of information).
It requires the genitive case:

• у администратора = from the administrator / with the administrator / at the administrator’s desk

So у администратора marks the person you should ask/verify with.

The preposition перед (before / in front of) governs the instrumental case.
So отправка becomes отправкой (instrumental singular):

• перед (чем?) отправкой = before (what?) sending

Here отправкой is a noun meaning sending/dispatch (a verbal noun). Such action-nouns often take a genitive “object,” similar to English sending of (a) document:

• отправка (чего?) документа = sending (of what?) the document

So the structure is: перед [sending] [of the document].

Yes, and it changes meaning slightly:

• перед отправкой документа = before sending the document (one document, context-specific)
• перед отправкой документов = before sending documents (plural / multiple)
• перед отправкой этого документа = before sending this document (more specific)

It’s not an imperative form; it’s a recommendation. It’s polite and fairly neutral, suitable for instructions, emails, or customer service:

• Softer than Уточните адрес… (imperative)
• Similar in tone to Вам следует… (you should / you are supposed to), though следует can sound more official.

Some reordering is possible, but it affects emphasis and naturalness:

• Вам стоит уточнить адрес у администратора… is the most neutral.
• Адрес стоит уточнить у администратора… emphasizes адрес (topic = address).
• Стоит вам уточнить… is possible but can sound more literary/marked.
• У администратора стоит уточнить адрес… is understandable but less natural; it can momentarily sound like it’s worth it for the administrator to clarify the address until the listener re-parses it.

Yes, if the context already makes it clear who the advice is for:

• Стоит уточнить адрес у администратора… = It’s worth clarifying the address… (general recommendation)
  Including Вам makes it directly addressed to the listener and slightly clearer.