Pondremos rosas en ese florero antes de que lleguen los invitados.

We use pondremos because it’s the simple future tense, which

• expresses a planned action in the future,
• sounds more formal or decisive than vamos a poner, and
• is more concise than using the present when you clearly mean “we will put” rather than “we put.”
  You could say vamos a poner rosas…, but pondremos puts a little more emphasis on the “will.”

In Spanish, time conjunctions that anticipate a future action—like antes de que, cuando, hasta que—trigger the subjunctive because you’re talking about something that hasn’t happened yet.
Here:

• antes de que = “before (they) ever arrive”
• lleguen is subjunctive, matching that unsettled, future event.

No, because:

• With an infinitive after antes de, the subject of both clauses must be the same (“we” in your main clause and the subject of the subordinated action must also be “we”).
• Here the subjects differ: we (pondremos) vs. the guests (lleguen), so you need antes de que + subjunctive.

Spanish demonstratives:

• este florero = the vase close to the speaker
• ese florero = the vase close to the listener or just mentioned
• aquel florero = the vase farther away from both
  Using ese suggests you and your listener already know which vase you’re talking about, but it’s not right next to you (so not este) nor very far away (aquel).

Yes. Both mean “guests,” but:

• invitados emphasizes “people invited” (focus on invitation)
• huéspedes emphasizes “people staying/visiting” (focus on hospitality)
  In many contexts they’re interchangeable, but subtle nuance differs.

Spanish word order is flexible. Both are grammatically correct.

• lleguen los invitados puts focus on the action “arrive.”
• los invitados lleguen emphasizes “the guests.”
  Choose the one that fits your emphasis; the first is slightly more neutral.