Furthermore, replacing all the planes would take many decades, while synfuel kerosene might go into production much sooner.
Do the hydrogen airliner boosters take all that into account?
Sure, and other things too, like combustion benefits and hazards. But regarding your money worries - they don't need to happen overnight, just like electric cars don't replace gasoline ones suddenly.
Hydrogen fundamentally more potent fuel than kerosene - both by energy density and heat characteristics. Like with other commercial technologies, to be accepted it has to change traditions somewhat, which isn't easy. Safety is another important matter - it remains to be seen if hydrogen can be safely brought onboard to regular commercial liners, though advancements demonstrated by Toyota Mirai remind that progress doesn't stand still.
I wouldn't mind a future where we use syntin-based fuels or similar for aviation and push for max renewables and nuclear for power elsewhere.
Not really. Most of commerical aviation doesn't use leaded fuel anymore, and general aviation uses leaded gasoline because it is technologically still mostly stuck in the sixties.
Replacements for currently used anti-knocking agents are searched for, but less so specifically because most commercial use involves Jet-A1, and low-end general aviation uses MOGAS where possible due to general lower prices (though those tend to vary - sometimes AVGAS 100LL can be wildly cheaper than MOGAS)
Remember, aviation fuels need to work way above 10km above sea level.