Let's say i've done my EV training right and maxed out each of my two chosen stats at 255. Does this mean I can now fight any Battle without worrying about gaining unwanted EV points anymore?
Yes. Once a Pokémon has reached 510 total EVs, they cannot gain any more through battling (unless you get rid of EVs by using certain berries).
Upon maxing out, what does this mean for stat gain? Will EV points no longer be counted at max value?
As you stated in your question, it's proportional to the Pokémon's level. A lv 100 Pokémon will always gain 1 point for every 4 EVs in that stat (assuming the nature doesn't interfere), while a lv 50 Pokémon would need more EVs for them to make an impact in the final stat, but the EVs gained will be reflected through stat growth.
If I give another Pokémon the Exp Share item and use my maxed out Pokémon for the battle, will it still gain EVs? I know even at level 100 where my Pokémon would no longer gain exp, the exp share still gives half to the holder of the item, and as long as it still gains Exp it would still be gaining EVs too correct?
The EXP share (and the game in general) rewards both EXP and EVs independently of each other; a Pokémon with maxed EXP (lv 100) can still gain EVs and vise versa. So say for example that you have a Pokémon max EVs battling and another Pokémon without max EVs in your party with the EXP share. The first Pokémon only gains EXP, since its EVs are maxed, but the second one gains both EXP and EVs.
I'd also like to note here that unlike EXP, EVs are not divided among Pokémon when more than one battles; if two Pokémon are used to defeat a Pokémon that gives 2 EVs, they both get 2 EVs (assuming neither of them are maxed out).
Hope this helps.