Questions about ROMs and Porting

So, Since the Cyanogenmod source is so big (I heard that it has 19GB), people like us who have gay as slow internet have no chance of even trying to cook a ROM for our unsupported devices.

Since I can't download the whole CM source and I see awesome ROM's out there, I Ported one ROM called Cyanfox (which was made for Samsung S2 i9100) to Samsung Infuse 4G using Beanstalk ROM as a base. I followed a guide which asks me to replace some folders and edit some files. It was walking fine. Then came an update to both the ROMs, Thinking that cince it is a update, it will have the updated apps and source, I tried porting them again but this time Beanstalk updated ROM as a base cause a bootloop while the Beanstalk old ROM works fine as a Base ROM.

1. Now what I don't get is, why is Beanstalk new ROM not working as a base? (Since the new Beanstalk ROM will have updated Source, I want to use that ROM as my base.)
2. And the second question is, what files does actually matter when you are Porting or cooking an Android ROM? (I mean like windows works on all kind of PC's all it needs is the drivers right? The drivers actually matter on windows)
3. And the 3rd question is can I update the source files somehow without downloading the whole 19GB CM source and if possible how?
4. The 4rth is, I heard from somewhere that boot.img is the file which separates the phones? Then why can't we just port ROM's by replacing boot.img since these two phone's are from the same vendor and it has likely specs. (Which is not possible I think)

I am really really a newbie to this thingies, I just wanted to clear out these things. Sorry for my noobness.

Thanks!