Regarding your question about which Modern Greek bibles are used mainstream in Greece among different churches (that aren't traditionally reading the original) - you may find that most Orthodox Christians simply don't read the bible in any language.
Bible literacy among the laity is not generally promoted by the Orthodox Church, though it's not actively discouraged either. It's widely believed among these communities that interpretation of biblical texts is best left to the clergy. And the clergy sticks to the Orthodox Church's official version of the "original" Greek text - codified in the 1904 edition of the New Testament Patriarchal Text (AKA The Antoniadis Text). This is a Byzantine text with some ecclectic readings.
The history of Modern Greek translations of the Greek bible is quite interesting and complicated. The history is rife with scandals, since the concept of paraphrasing the "original" holy text in the modern vernacular was looked down on for a very long time as a "corruption" of the Holy Writ.
The first version to be published, that I'm aware of, was Maximos Gallipoli's Vernacular Greek NT (printed in Geneva c. 1638). It was reportedly translated about 10 years earlier, per the initiate of Patriarch Cyril Lucaris of Constantinope, who was later strangled!
According to Wikipedia, "Since Patriarch Lucaris had been strangled Maximos' New Testament was hardly used". That explanation seems quite inadequate to me. I would guess that the base text for this version was probably the Textus Receptus (TR), since it was printed 5 years after the 2nd edition of Elzevir's Bible.
Seraphim of Mytilene''s Modern Greek New Testament was published in 1703, and quickly condemned by the Orthodox Church in 1704 - specifically by Gabriel III of Constantinople. Seraphim was exiled to Siberia (presumably for his role in the production of the translation). And in 1705 Gabriel III forbade Greek students from studying in London (again, presumably, due to the connection with this unsolicited Modern Greek translation).
I imagine Seraphim's version must have improved upon Gallipoli's version or been based on it. Alternatively, if Gallipoli's version had been suppressed somehow, maybe Seraphim started from scratch. I'd really love to know more about this whole history. But it's a mystery to me, since I've never seen either version and I haven't been able to find much information about them.
The next translation into Modern Greek was Vamvas' New Testament (New Testament published in 1833 / Old Testament published in 1850). It was based on the TR and written in a form of Katharevousa that was extremely close to the original "Koine" Greek. It reads more like a paraphrase than a translation. It was not well received by the Orthodox Church. But it's become the official version of the Greek Evangelical Churches. So, to answer the original question - which Modern Greek bibles are used mainstream in Greece - it's Vamvas.
The turn of the 20th century saw the near-simultaneous release of 3 Modern Greek translations of the Greek New Testament that had significant political consequences. They were the cause of the Gospel riots, which resulted in an official ban against Modern Greek bible translations. More about this can be read on the Wikipedia article about the Gospel Riots:
First there was the "Anaplasis Paraphrase" of Matthew in 1900. This Katharevousa version inspired Queen Olga's Translation of the Gospels (by Ioulia Somaki; edited by Prokopios, Pantaazidis & Papadopoulos), which had the periphrastic (translation) text printed on the opposite page from the original "Koine" Greek vorlage.
In that same year (1901), Pallis' Evangelika (a Demotic Greek translation of the Gospels), which was quickly condemned by the Orthodox Church. In a sharp departure from tradition, this edition was based on the emerging Critical Text, not the Textus Receptus or Byzantine Text-Type. This edition is actually available in print, unlike all of the other aforementioned versions (as far as I've aware). It was reportedly wholly independent from Queen Olga's endeavor, with all of the translation work and publication taking place in London. However, the general public (due to complete ignorance and/or conspiracy theories) conflated Pallis' version with Queen Olga's translation, which instigated the famous "Gospel Riots" - which resulted in the deaths of several civilians.
The next Katharevousa translation/paraphrase, published in 1967, was Vasilios Vellas' version, which was derived from Vamvas.
The first complete Demotic translation of the whole bible was Today's Greek Version (published 1985 and updated in 2003), based on the official Antoniadis Text and approved by the Greek Orthodox Church.
The next Demotic translation was the Spyros Filos' Bible (in several editions from 1993, 1995 - the 1st monotonic edition, 2008 & 2013).
Another Demotic version is the Antonis Malliaris (Malliaris-Paideia), which was published in 2012 and approved by the Greek Orthodox Church. This version is an extremely liberal, loose paraphrase of the original Greek text.