Any advice for a potential iPhone defector?

Hello, I need some advice from Android people in the know.

I am currently an iPhone user but I think I intend to repent and change this.

At the moment I have an iPhone 4. My contract was up ages ago and I intended to wait and get a 5, but the 5 pretty much failed to wow me when it was announced and I've been wondering what to do ever since. I think I'm going to wait till WWDC just incase they announce an iPhone 5s and iOS7 that change the equation, but even if they do, I still don't expect I'll bite. I just don't like the form factor of the long thin phone too much and android seems to have a lot more going for it these days than it used to. Not least all those phones with massive sharp screens.

I'll probably end up getting a Samsung Galaxy S4. All the reviews say that the HTC One is amazing to hold, but they also say the battery life is an issue, and so I think the S4 has the edge. The S4 also has that ability to let me watch a video and browse the web or sit on IRC at the same time (I believe you can either pop the video out or run the screen as half and half), which in my mind is a pretty killer feature. I live in the UK, so a lot of the american podcasts (read: the Vergecast) and whatnot that I like to stream are at odd times for me, so I have to watch on my phone when I'm on the go. I'd like to be able to still use my phone whilst watching a video.

I have a few questions I hope somebody can help me out with:

1) Everybody in the tech press (Verge included) seems to live on the mantra that stock android is better than, say, Touchwiz. From my googling it seems like that is mainly because Touchwiz used to be bloated and resource hungry, but it seems like that isn't such an issue anymore. I understand that stock phones get updates faster/at all, but is there ever really all that much in the updates that are a huge deal? Essentially, why is stock android so desirable?

2) Like I mentioned above, the watching video while doing something else is a huge draw for me. It's what's really tipping the scales in Androids favour. Is this doable on stock android? I know for instance that a stock version of the S4 is due to be released next month, so if the answer to question 1 is "yes, stock is better because x, y & z" AND stock lets me watch and surf as well then I might wait and get a stock phone. Or maybe I'd buy the nexus 4 and save some pennies.

3) I'm actually a software engineer by trade and I did a bit of JAVA at university. I understand Java is the language for android apps, but I also hear peole talking about it being preferable to develop on stock android if possible. I assume if I want to tinker and build apps for android I could still do this on a Touchwiz skinned device? What/if any are the limitations?

4) Lastly, the android community seems big on rooting phones and installing custom ROM's and all the rest. One of the reasons that I stuck with my iPhone for so long was that it was nice and stable and didn't cause me a lot of headaches. At the time I bought it that was not the case with Android. I believe that has changed recently, but the fact that even main stream reviews occaisonaly talk about modifying devices and installing custom firmware makes me wonder if I'd need to tinker and maintain an android phone to really get the most of it. Is that the case or is it just that it's easier to mess with android so more people are into it as hobbyists? I have so much other stuff I need to worry about day-to-day that I'd like an android phone that (to use presumably heretical language) "just works".

I'd like to hear your advice or anything else you think I might need to know. Hopefully I'll be an android user soon and wondering why it took me so long to switch.

Thanks in advance,