1) yes anything is possible, look at the mac mini, 2011 had a quad core cpu in there. if you haven't noticed, Apple tends to the impossible.
2) they can co-exist, til apple sees a drastic drop in sale on the rmbp 13, and eventually not worth making them if people dont buy them (why do you think the 17 isn't around anymore, people stopped buying them!
3) Yes, 1" space to apple is 2 acres of land. they have done marvels with space in the macbook air. imagine what they can do with adding another 1" into the rmbp 13.
4) another reason why 1" of space will only greatly benefit whatever decision apple decides to cramp in there. "oh how about we put a discrete gpu in there", giving the consumers what they have been asking.
but hey, each to it's own.
1) no you dont understand the proposition of each product what they offer to each user.
2) they havent done anything in that sense, cooling is about heatpipes, fans and airflow. Interesting constricted airflow is much better for cooling, guess what a chassis that is slim can do.
3 and 4) I dont see apple putting a dgpu in the 13, they never did that in the mbp line.
What you dont see is the difference in focus of those 2 products, they overlap, sure, however one was designed with a complete different set of requirements than the other, and given that the new chassis for the mba approaches do you really think that they will make something that can handle quads? please.
The problem is that you believe too much on whatever you think you believe, and dont face what we have here to make what you think/desire is going to happen or not. Skylake is your best bet on whatever pipe dream you are on, and we dont have any info on that.
and you know the most probably line to disappear? mba. ultrabooks may be all the rage right now, but they weight the same as the rmbp, some less, some more, however they are not as powerful as the rmbp can be. So why keep manufacturing one machine that really has very low margins? The cpu in there cost the same as some non oem quads, the i7 model? almost the same price as the 3840qm, SSD standard? and the list goes on.