IMHO I don't think money laundering fronts are necessary or even happening in Kenya.
First, you can use paper money to buy anything and no eyebrows will be raised, so you can keep millions in cash under your bed, spend at your own pace, and nothing will happen. We saw this when NYS scandal broke and the perps had millions just lying around their houses. The only case I can remember where someone got in trouble is when some idiots walked into a bank with billions in fake currency.
Kenyans are risk averse. We prefer liquidity over assets. Beyond kujenga nyumba ushago, (dead capital) if we have an excess amount of money, it will most likely remain cash.
Most importantly, KRA is poorly structured. They have limited ways of monitoring how much money you spend vs the income you declare.
Businesses congregating in one section is one of those weird quirks of human psychology, I wouldn't read too much into it. We prefer kuenda hivi na kurudi when shopping. It happens from the smallest businesses to the largest, prostitutes flock to one street, bodas and taxis tend to park in the same spots, big firms build offices close to other big firms.