Regarding your question about the letter of explanation regarding your split-residence with your spouse, this is a fairly common living arrangement and it is common for lenders to ask borrowers to explain if you two are living as a common household with shared income and expenses or are married in name only.
Regarding your question about the townhouse, I cannot offer a helpful opinion. You stated that you like the area and the design of the townhouse. Obviously, you are willing to accept the risk that the builder may never start the project or even stay solvent. What is a "reasonable" time to wait is a question I cannot answer because I do not know the market in question. In some markets today buying anything but a short sale property or foreclosure is foolish, but in other markets the housing market is much healthier.
I hope this information helps you Find. Learn & Save.