In the OTU by 1105 drop tank equipped merchants have been used for over a decade in the core sectors of the Imperium.
In the OTU drop tanks are single use if they are dropped.
Remember the OTU is not perfectly described by the rules as written, the rules are modified to describe the OTU setting. If the setting contradicts the rules change the rules to match the setting. The possible exception to this are the rules in T5 which describe MWM's vision of the OTU, but even then there are contradictions between his novel and the rules, and he has mentioned during interviews setting material that isn't in the T5 rules as yet.
As to how the hydrogen is used by the jump drive during jump, the original article said this:
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When the jump drive is activated, a large store of fuel is fed through the ship power plant to create the energy necessary for the jump drive. In the interests of rapid energy generation, the power plant does not work at full efficiency, and some of the fuel is lost in carrying off fusion by-products, and in cooling the system.
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