So here are your questions, answered in order;
1. How nice is a zero stop feature. I know it's not necessary, but is it something you wouldn't live without, once you have it?
The zero stop is exactly as you imagine. Not necessary, but nice to have. For the most part, you won't really use it when your hunting, because you'll be within maximum point blank range when you're hunting (probably) and aren't going to make in-field adjustments. You'd potentially make use of the zero stop when you're setting or testing your zero, but otherwise your adjustments would stay the same. Simply put if you're not adjusting your elevation, then you will never need a zero stop.
You also mentioned you'll be plinking long range later. Here is where you may potentially use it, but again not "needed". I think the true utility is in quick multiple variable distance targets where you'll have to go back and forth over a large elevation range, where you could potentially get 'lost' in the dials, but even when target shooting, it often isn't the case.
Lastly, for what it is worth, I have a zero stop on my scope, and I use it on occasion to give me a warm and fuzzy feeling that I'm on the right rotation, despite the fact every logical thought knows I am, I sometimes doubt myself.
2. I have no interest ranging with a reticle, and like a fine cross hair. This means second focal plane would be better for me, right?
Well, firstly the reticle. Perhaps yes, you may not be ranging with the reticle, however if you begin to plink at long distance with your future .223 you'll be up against wind. While yes you can dial to correct for the wind, you'd be dialing the average wind, and it may be gusting. So, you can't be dialing the wind for every change from the average windspeed, so you'll hold off to correct for the change from what you've dialed and shoot. Now you can gut feel the correction with a cross hair reticle, but it would be much wiser/accurate to estimate the wind change, determine the correction in moa and hold off on the reticle and shoot. So while no, you may not ever need a ranging reticle to range distances, it will certainly be helpful for on the fly wind corrections.
Second Focal Plane and thin reticles. Firstly on this, I would personally dis-associate the correlation between second focal plane and thin reticles. Simply because it isn't always true. I would rather evaluate the thickness of the reticle on a scope vs another scope based on what the reticle subtensions are listed in the product manual. I'll address your particular riflescope specifically in a later question.
As for reinforcing the difference between first and second focal plane in terms of reticle thickness at given magnifications,
Second Focal Plane : Second focal plane reticle thickness doubles in moa/mil when you drop the magnification in half. So if you are at 16x power and your reticle is .1moa thick, at 8x power it will be .2 moa thick. (Sidenote: Not all labels for the magnification power line up perfectly for the "actual" power, ex. 8x label may be pointing at 7.5x)
First Focal Plane : First focal plane reticle thickness remains constant at whatever magnification you are at. So if you are at 16x power and your reticle is .1moa thick, at 8x power it will also be .1moa thick.
3. How "fine" is the ebr-1 Moa reticle?
I will refer you directly to Vortex's Website which lists the EBR-1 MOA reticle subtensions on this
diagram. Specifically the thinnest line is .1moa thick at maximum magnification / 16x power. This means that it would cover up one tenth of a inch at 100 yards.
4. Is there a big difference between a 50mm and a 56mm objective?
Unfortunately all I have is here say on the subject as I only possess at 56mm objective scope. With that said, from what I have heard it is of little difference and allows the image to appear marginally/potentially insignificantly brighter at higher magnifications. I would not get caught up in which size objective is right for you, but rather that the scope is of quality, and contains the feature set you are looking for.
5. I've never had a scope with an illuminated reticle before. How useful is it during broad daylight? Or is it mainly for use in low lit situations?
I have a red illuminated reticle on my scope, and I have not personally seen any "utility" to it use in broad daylight, although you can certainly see the red in the daylight. If anything the reticle would appear less crisp, providing less defined lines in the reticle due to the glow of the red. However it does look cool, in my opinion anyways.
As for low lit situations, I can absolutely see the utility, here you do not have to put it on as high of power as you would during the day, so you don't get the poorly defined line glow I mentioned earlier, and helps the reticle not get lost in the dark, especially if you have a fine reticle. However this feature again is not useful if you do not find yourself in low lit situations.
6. I think that's everything for now. I look forward to your response.
Hope it was what you were looking for, and I hope it helps you out. Besides answering your questions, I thought I'd say good choice on the moa reticle, I have one as well. I find that despite living in a metric society, I still estimate lengths and distances in imperial, and the mental math is much easier using moa for that. Additionally there are a lot of ranges that are still measured in yards as well. Just my 2 cents anyways.
Cheers.
