Have these countries reached long-term equilibrium, in which approximately 3/4 of the
  people live in urban areas & 1/4 in rural area?
In the United States long term equilibrium has has been achieved. Around 80% of Americans live in urban areas. However, China is a long way from equilibrium. Nearly 70% live in rural areas there. In the United States people migrate into and out of cities at a relatively constant rate. In China, rural workers are swarming to the cities, against government regulations.
Will counterurbanization resume in the future b/c people prefer to live in rural areas
I think that the rural-urban migration is a part of the demographic transition cycle. Undeveloped countries are mostly rural and agriculture based. When a country first industrializes, the urban population skyrockets. When the urban population reaches about 75% the urban movement levels off with people moving into and out of cities. In the late stages people begin to move back to rural areas because cities have become overcrowded and less pleasant to live in.
Is the decline of the rural economy reversible?
When people begin to migrate back to rural areas the rural economy will begin to revive. If more people are living someplace, they will buy more goods from the local economy and help rejuvenate it. For this reason I believe that the rural economy will have a comeback.