It depends on the architecture chosen. Together we define what can go through an external API and what must stay on your servers. Fully in-house hosting is possible for sensitive data.
With measurement and a single channel used well. A business that masters one channel perfectly gets better results than one that is half-present everywhere.
The ones where your customers are. For a B2B business, LinkedIn is often the priority; for a local shop or a visual product, Instagram and Facebook are more relevant.
Two to three genuinely different directions, each with its rationale. We prefer three well-developed directions to ten superficial variations.
UX is about the journey and the logic: can people easily find what they are looking for? UI is about the look: colours, typography, components. A good project handles both, in that order.
For a site with an admin area (WordPress, online shop), yes: plugin vulnerabilities are published and automatically exploited within days. A static site with no admin area needs far less attention.