1. But official closure of the United Arab State project caused a change of focus towards the USSR, as the only power in the world capable of providing effective assistance to Yemen in their existence as a nation state, as the USSR could provide the necessary statehood resources/set of competencies needed for the PDYR to function independently. Or “...nation state. The USSR could...” [which one is better?] Both are correct but the sentence is getting to be a run-on sentence. I would use "...nation state. The USSR..."
2. In 1986 the YAR (Yemen Arab Republic) labour resources constituted (of) 1.6 million people, including Yemen immigrant workers. [do I really need ‘of’ there?] No, better without.
3. In the PDRY (People’s Democratic Republic of Yemen) the economically active population in 1988 consisted of 435,000 people, of which 11.7% were jobless, 52% of the economically active population worked in agriculture, 14% - in manufacturing and 21% - in the services sector.
4. In 1990 in the United Yemen more than 61% of the population worked in agriculture, about 22% - in the services sector, and 17% - in manufacturing.
5. Yemen is one of the poorest Arab countries. The GDP (Gross Domestic Product) per capita in 2009 was $2,500 (among Arab countries only Sudan and Mauritania have less and overall it is the 173th place in the world).