1)
Why was the tax given to Elazar and not to the other Kohanim?
1.
Oznayim la'Torah" Because the Kohanim are obligated to make sure that the Kohen Gadol is wealthy - and by giving the Kohanim's portion to Elazar, it was considered as if they had fulfilled that obligation. 1
1
As the Gemara states in Yoma 18 'Gadleihu mi'shel Echav'. Refer Emor, Vayikkra, 25:10:0.1:1.