In view of your line achieving somewhere around 2-3Mb tops* then you probably did the right thing in signing up for the 2Mb account.
*The problem is those are just estimates and it does depend on your own particular line... which until it goes live and you can get some linestats from your router we cant really say for sure.
Re the splitter, Ive used and know of many configs this way where it doesnt cause any problems at all.. however if speeds are slower than anticipated then it is one of the areas to look at.
If I was in your position then I think I would have done exactly the same thing as you have done and only sign up for the 2Mb account and save yourself some money for doing so. Until you know for sure that your line is capable of much higher speeds then it may not be worth while investing time and money into correcting something that may only give a slight speed increase.
If you have probs attaining the 2Mb, then the internal wiring situation is something that should be looked out.
IMHO you are probably best waiting until you are connected.. then once you are if you post your
linestats, then we will be very happy to look at them for you and give our opinion on the condition of your line.