This entire argument is a load of festering feculent crap. And I do not mean that as a personal attack, but this "rebuttal" is common on the Internet amongst those whom are not aware of their own prejudices and hypocrisy.
R'as Al Ghul has been portrayed one way since 1971, yet no one seemed stirred to complain about an Irish actor portraying this otherwise Arabian man. When a French actress was cast to play Talia Al Ghul, a Arabian woman, no one was sore about how it would be a drastic change to a big love interest for Batman.
In the Spawn film, Chapel, a Black male, became Priest, a White female. Terry, Al Simmons' best friend, was changed from a Black male to a White male. Was anyone pitching a fit about that? Why is it that people complain when White characters are changed, but when non-White characters are changed, the arguments and rebuttals are non-existent. These are fictional characters. their ethnic composition doesn't matter unless it is integral to their characterization (e.g. The Blue Marvel, Colossus, Kitty Pryde). Nothing about Mary Jane Watson requires her to be White. She could be Asian and could still be a party girl. She could be Black and still be a gorgeous model. Whiteness has nothing to do with WHO MJ is as a person.
So, let's see what you're saying...
You bring up R'as Al Ghul and how Liam was cast and he's not an Arabic. Ok,l lets look at pictures:
And now Liam's portrayal:
Actually, looks like they did the character justice. Although Arabic R'as wasn't dipicted as dark skinned in the comics from what I'm seeing, and since Liam didn't play this character with an Irish brogue, I think hey did good!
Next you go the same route using Talia. Ok, again, let's look at the pics:
And now Marion's version:
So again, I'm not seeing a problem. Looks to me like the casting director got the look down pretty well. And again, it's not like this French actress was using her French accent.
Now you kind of go off into "who cares' land with Chapel from Spawn becoming Priest and Al Simmon's best friend.
You may not agree, but there is a huge difference between Spiderman and Spawn. Especially for the GA. The GA knows who Spiderman is, and generally even knows who MJ is. The average GA would have no idea who or what Spawn is. So really, this is a bad argument.
Now, when Nick Fury was changed, there may have been a little bit of rumblings, but for the most part it wasn't that big of a deal. Why, because Nick Fury is (or at least was) kind of peripheral. Spiderman is not, nor is Gwen or MJ. The look of these characters matter, and that's what this argument is about, their look.
Many of us had issues with Dunst's MJ and it, of course, had nothing to do with race, as it doesn't now. It was look. That's why a Nikki Cox (back in the day) would have been great. Because she had the look. Why did so many of us have issues with Shaelene? Because of the look they went with when filming. Again, nothing to do with race. Now, people like you, are trying to turn this into a race issue when it's not about race.
Again, it's solely about look. MJ needs to look like MJ, and that means an actress that looks like this:
And not this:
or this:
or this:
or this:
or this:
If she can hold her own with Leo, she can hold her own with Andrew, but yes, the chemistry would have to be there, which is something that you can only tell by having actors read together. 
