No need to go point by point. Utilisation can mean anything you want it to. If more people start using Bitcoin and buying and selling more goods the number and sum value of transactions will increase. This sounds like a good definition to me :-)
An increase in the number of people using the Bitcoin Network do not mean there will be more transactions or less transactions. What increases or decreases the volume of transactions is relative to what people decide to do with the BTC they own. So there is no direct correlation between the number of people using the Bitcoin Network and the volume of transactions.
You can create SatoshiDice examples which work the other way and use that prove your point if you want.
...and I could create a script which could raise the number of transactions by sending BTC between keys I own. That, in accordance with your sound definition, would mean that the number of people using the Bitcoin Network had increased.
Fine, so do not make statements of "percentage" when there is no "tools to calculate the percentage of transactions which include a fee". This show that you cannot prove the correlation between the increase of Bitcoin users and the increase of transaction fee volume. If you do not know the "percentage" of transactions which include a fee, you are not in a position to assert that "sum value of transactions will increase" just because there is "more people start using Bitcoin and buying and selling more goods".
Looking at a recent block, you can see people/machines have different ideas of a reasonable fee:
http://blockchain.info/block-index/426239/0000000000000004604489e4e1788c16479c60c37f55671ddca4cf2706915a95Generally less than 0.0001 BTC by my eye-balling. Pool operators do whatever gets them the most miners. If they started advertising their mintxfee settings, then miners could make a decision about which pool to join based on that. I'd personally join a pool which encourages a higher fee than the status quo with the long term goal of raising fees.
Payment gateways generate transactions and can add a fee to a transaction, I think you misunderstand at which point the fee is added. as another example, some of the exchanges already charge their own withdrawal fees, but you can guess that they aren't adding a percentage of that to the transaction fee, which perhaps they should :-)
Payment gateways do not generate a transaction, that is done by the Bitcoin Network. So payment gateways cannot add a fee to a Bitcoin Network transaction. What a payment gateway can do is use the Bitcoin Network to pay out they customers. Then the Bitcoin Network generate the respective transactions and a potential fee is added. The transactions fee in discussion is under strict control of the Bitcoin protocol, which is under control of the developers, pool operators and solo "miners".
Now, in relation to the topic subject, if the buyers of the KnCminer products expect they will be able to achieve a positive ROI based on the increase of fees, whatever is volume or price, they are completely screwed. An increase in the price of a transaction fee would result in less transactions, since common sense dictates that people try to pay less fees as possible. A significant increase of the volume of transactions would only be possible if all sectors of the global economy were using the Bitcoin Network. That is not the case,since only the third sector use the Bitcoin Network.
The KnCminer customers can only expect that difficulty will not reach an exponential increase before the next halving of the block reward. But this is unlikely to happen, since the tendency is more and more hash rate being added to the Bitcoin Newtork.