When did Daniel Lencina become Lithuanian?
Very recently. I got the official word a couple of weeks ago.
Is the Kevin Sorensen (GER) on the ranking list the same player who played for Ireland in this years Davis Cup ties?
Yes. This is another case of a Greg Rusedski type national jump. As you can see they are quite common. Sorensen plays for Ireland now. Oddly his brother, Louk, is still listed as German.
Could the player "formerly called Segodo" play for Qatar or Benin in future Davis Cup ties? What happens if the two countries ever meet - could he play himself?
No, you can only be eligible for Davis Cup under one nation. If Segodo/Saeed ever plays Davis Cup again, it will be for Qatar, he will no longer be able to play for Benin. But he has quite a few brothers that still do.
Is Alexander Tabares, ranked last year, Spanish or Cuban, or are there two different players with the same name as with Francisco Cabello?
The Tabares brothers are quite an interesting story. Like many Cubans they fled the country. Alexander settled in Spain while older brother Mario became an American citizen. Check the player file, it is always up to date with correct spelling and such and often contains notes.