Many of these issues are not specific to Pak128.Britain-Ex. Changing the routefinding algorithm to check why a convoy cannot reach its route would be highly complicated to program and would substantially increase computational effort; nonetheless, there is already a system that will give an indication that the weight is too high for a route in at least some cases.
The changing of the title bar colour is deliberate, and is also in Standard; it is there to show which window has focus. The towns with few buildings are caused by non-optimal settings in the city generation dialogue; you should increase the median population size of towns.
As to the staging post and 'bus stop, they only share a graphic because nobody has yet had time to give the staging post a dedicated graphic. The demo.sve issue with the cobblestone road comes from the fact that that particular demo.sve comes from Simutrans-Standard in which there is no weight limit.
Thank you for your feedback, however; let me know your experiences and enjoy playing!