To properly size products for structural capabilities we would need more information on all loads and applications. Please consult a design professional to assure all local codes are met.
Yes but it's not treated. Keep it painted.
Though untreated Douglas Fir 4x4 posts are not suggested for exterior exposure applications, they are a structurally stable, durable lumber product which, assuming allowed to properly dry and then surface-sealed (with regular maintenance) are suitable for limited outdoor applications where significant water exposure can be avoided; untreated DF is not suitable for ground contact applications.
No, it is not.
post base connector
If possible use the same screws that came with couch
no not pressure treat
You could use them but keep in mind they are not treated and make good termite food.