So what happens if I max out the Sp. Def but keep fighting the pokemon that drop the sp. Def ev's?
Nothing happens, once you've got max EVs in a stat (255), it wont increase any more.
Darn. Was worried about that, though I still should be ok, right, if Hoothoot didn't drop any EV's that would hinder my Umbreon's defense stats? Like I said, I don't know what types of EV's it gives. And hopefully, if I'm EVing right- there should be 6 EV's leftover to allocate to any random stat after I max out 2 others with 252 each, I've read.
Hoothoot gives 1 HP EV, if your Umbreon was still holding the Macho Brace at the time then it would have gotten 2 HP EVs. Whenever your Pokemon gain experience they gain EVs, only exception being if that stat already has max EVs.
How would I determine Umbreon's EV's/ IV's in HG or SS? I tried doing as much research as I could, before I attempted to train it; and got a good grasp on what nature I wanted to look and breed for- which was very time consuming- and the stats that were most beneficial to that nature, along with which pokemon dropped the EV's I was looking for to boost the stats i wanted, but there's still much I don't quite get, so I appreciate you taking your time to better explain.
You would have had to be counting all EVs you've received. If you've only been fighting Tentacruel, Tentacool, Mantine and that one Hoothoot then, depending on how many Tentacruel, Tentacool and Mantine you've fought you probably have 255 Special Defense EVs and 2 HP EVs.
You can use this calculator to find IVs: http://www.serebii.net/games/iv-calcdp.shtml
And I would recommend you read both of these pages on EVs and IVs.