"It is necessary to provide a legal basis for AN economical completion of investments..."
"An" is used here because we are not talking about a specific "economical completion".
"It is also important to introduce (ZERO) instruments of innovative procurement.
… an increase in employment and (ZERO) reduction of structural unemployment."
In front of "instruments", we don't need an article because "instruments of procurement" is a standard phrase and we don't know what specific "instruments" will be "introduced".
In front of "reduction", I would use "an" because previously it reads "an increase"
Potential for THE long-term sustainability of the projects funded is high. A friendly administrative environment will enable THE easier establishment of businesses.
"THE long-term sustainability of the projects"...The (specific) projects have a specific long-term sustainability so "the" is used.
A friendly administrative environment will enable THE easier establishment of businesses.
This company has a specific plan for an "easier establishing of businesses" so "the" is used.