Nope. The Elders have responsibility within their own congregation, and potentially over any congregations they plant through missionaries until those "child" congregations become mature and have established, qualified Elders of their own.
Yes, missionaries are transitory. That is why they establish Elders within each congregation to lead that congregation when the missionary is no longer present.
Never said the Elder was the same as a deacon. The Elder is the same as the Presbyter. But the Deacon is a separate office, subject to the Elder. The deacon does more of the hands on work of serving physical needs of the Church, while the Elder does more of the Spiritual work of teaching the Word and guidance within the Church. This is demonstrated in the early Church in Acts 6:1-6.
pres·by·ter
/ˈprezbədər/
noun:
presbyter; plural noun:
presbyters
- historical
an elder or minister of the Christian Church.
Episkopos (ἐπίσκοπος) is the Greek word for an "overseer" or "superintendent," a term used in the ancient world for government inspectors and in early Christianity for the spiritual leaders of local churches. The English word "episcopal" derives from episkopos, referring to a church governed by bishops.
Again, this is the same office. The same office within the Church, according to Scripture, is also called the Bishop, Elder, Presbyter, Overseer, Shepherd, Pastor.