Rachel,
The given name Yehoshua (Joshua) (you noted your document has the spelling Yehoshue) doesn't translate to Jankel/Yankel in Yiddish, in a search of the JewishGen Given Names Database. Jankel is a Yiddish name for Yaakov (Jacob).
But a person could have had several Yiddish and secular nicknames. Does "Jankel" appear in any family documentation you have found?
A Hebrew name sounding similar to Yehoshua is Yeshayahu/Yeshayo (Isaiah). Those two names have at least one Yiddish nickname in common, though: "Shaye".
Mazel tov on breaking the brick wall between "Russia" and Kovno, Lithuania! I'm also looking for ancestors there, who seemed to have migrated back and forth from the former Kovno Gubernia to Latvia, making the puzzle more complex. I'm somewhat familiar with Silber/Zilber in the LitvakSig records, having come across the record you posted.
You probably know that name spellings could vary widely. We are reading transliterations based on input from the named person and the clerk who wrote them down.
Nancy Seibert
Rhode Island, USA
Searching: ALMAN/OLMAN, SALKOFSKY/ZALKOFF, TZIBERT/ZIBERT, SANDEL/ZANDEL, BURNOWITZ/BARANOWITZ, BLUM, BLUMBERG, VOLPERT, REYNHOLD, MATUSEWICZ in Lithuania, Latvia , and USA.