Wait. You quoted the rules, but then added terms that weren't there.
The section you mentioned doesn't say anything about speed dials. Just speed.
There's also nothing in that section (or engine techs) that prohibit spending a nav command. If there was an ability that let you execute the same command twice, you WOULD be able to use it during an ET move. (Which is why that ability seems unlikely to be added...)
For your last point, generally those situations play out formally like: "I have completed my attack step. Would you like to make any actions before I start the determine course step? No, ok, now I have set everything, spent my nav command, and am about to complete the determine course step, would you like to make any actions? Yes, you want to resolve g-8? Ok..."
To be honest, I would like to see clarification on if "before resolving the determine course step" means anytime during or before starting as you can make a case either way.
What speed your dial shows is what speed your ship is going unless a card specifies otherwise. Remember, card trumps rules.
Yes if there was a card that let you sped 2 commands separately you could do that. Since there is none and I don't think they will create one (far too much headache with that one I think) you cannot use a navigate command during an Engine Techs. It is just not possible with the cards out right now and the rules.