2- I do not know this for a fact, but I think it is rounded down, after all modifiers are applied.
1-I think I did use the one you are talking about, in the past. I found here a number of years back a better one. Better mainly as it could be used for army units battles and could handle some of the mods that provide more hit points.
3- well I do not agree with any of these postulations. First I think everyone that has played a great deal of AW will tell you an attacking warrior will fair far worse than 50/50.
If you have read many of the SG's played at AWE or better, you will often see them let that warrior attack their warrior, all other factors being equal.
I can tell you both from a statistical and a practical matter, the archer will not come close to 50%, even if the spear is not fortified and it will be most of the time. I ran a 100,000 battles and the odd only went to 34.2%.
Now understand that does not mean you will never see a string of archers killing spears. I have had them win 5 of 6 many times. That is just like any other event, std deviations tend to allow for runs.