Whereas reputable brokers are implementing various methods to...

Whereas reputable brokers are implementing various methods to prevent quote stuffing, checks on numbers of open orders, and number of canceled orders without executions, there are still a number out there that ignore such behavior that is representative of manipulative tactics.
In regards to gap filling, the unfortunate truth is that the current day algorithms and instructional texts in main stream are based on precepts that existed long before the present day electronic age. So traders follow such behavior - systems and traders that take a different approach are overwhelmed by the existing majority forcing them to play the same game. The trading community loves this since irrespective of the verisimilitude or lack of, they in effect get to set their own rules and ensure they are followed. In other words a self fulfilling system. The impact on stocks with low sp and low actual trading volume/order book depth is magnified and it is relatively easy to follow the herd to fill gaps but the statistical noise is equally magnified. Not all gaps are filled or require filling -microeconomic theory clearly indicates that with regards to the laymans term 'all other things being equal' and the econometric handling of that in terms of estimation of extraneous variables. As a simple example of gaps not filling, the 1970's actions of Opec whereby output was arbitrarily fixed. Or in more recent years the impact of subsidies and rebates on solar cell prices and demand in australia. Or the appearance of substitute products on the primary product market.
To infer all gaps must be filled is an assinine hypothesis and indicates little to no analysis of underlying pricing or market/economic conditions. And that is the way traditional traders like it and run the game. At the other extreme are the global commodity future traders where every piece of information digested and arbitrage is profitable.