Believe it or not, sails! Obviously you’re not going to get a 100% reduction because modern shipping companies wouldn’t tolerate being becalmed (and I’m not falling for that article’s “up to 90%” claim either, BTW – I only picked that one to link because it has a decent overview of multiple different technologies), but it can still make a big dent in the fuel requirements.
No. Absolutely not. Sorry, but I cannot this claim seriously at all. We are not going to switch to sail ships again. I don't think you grasp just how big modern shipping actually is, and how impossible such an idea really is.
I doubt you have any grasp of how massive the problem really is, and how tiny your proposed solutions are in comparison. For instance, you keep citing the possibility of using waste cooking oil for biofuels. Well, the world only makes 3.7 billion gallons of that per year: https://oilandenergyonline.com/articles/all/supply-and-demand-report-used-cooking-oil/
Converted to barrels of oil equivalence, that's around 100 million barrels. The problem? That's literally one day's worth of petroleum consumption: https://en.wikipedia.org/wiki/List_of_countries_by_oil_consumption
So you are about 0.3% of the way of solving the problem with that idea. Even if we could radically reduce petroleum use in the way you are imagining, that's still going to be a negligible impact. And the world's GDP is still growing. There's still multiple billions of people that will want to live like the first world. So demand for energy will skyrocket in the coming decades, not decrease. The problem will only get exponentially larger and harder to solve.
Ultimately, this is eco-Ludditism, and is more about wishing away the problem than actually solving it. Worse, you enabling the worse stereotypes about environmentalists. Namely that they are crazy wackos who aren't willing to engage with reality. Any solution must take seriously the idea that there >8 billion people on Earth now, and they all want to live in convenience.
The first point to make is that hydrogen is not decades off. Green hydrogen is happening now, and its production is rapidly expanding alongside the expansion of renewable energy production. Many sectors can rapidly adopt green hydrogen right away. This is similar to the conversation we we had about solar power about a decade ago. Critics of solar power back then were being Luddites (and sometimes secret fossil fuel industry stooges). They were convinced that solar could not be cost-effective or scale, based off of very outdated understandings of the issue, but they were wrong. This conversation is repeating with green hydrogen.
On a related note, pro-electrification crusaders are being hypocrites on this subject. They themselves are demanding that we wait decades for miracle batteries or multi-decadal long electrification programs. Because they want "perfect" solutions rather than "good" solutions. A good example is how they demand we fully electrify all rail, a process that will take decades, rather than doing something faster like switching diesel trains to hydrogen trains. In reality, adopting hydrogen now, alongside more reasonable forms of electrification, will be a faster path for reducing CO2 emissions.
Also note that most "fearmongering" types of argument against hydrogen originated from the fossil fuel industry. They are always spreading propaganda intended to undermine green energy projects, and make similar claims about all green technologies. Claims that hydrogen is dangerous, or a GHG, or will leak, etc., are all fear tactics created with minimal amounts of evidence. In reality, hydrogen has very few problems, and adopting it will drastically make transportation and industry safer and more green. It is unfortunate that many environmentalists have fallen for this tactic, but I suppose every green idea had to overcome it.
Finally, you can buy hydrogen and hydrogen-related products. Sure, we are still a bit early on the adoption curve, but that is true of every new idea. Someone can buy a hydrogen car, or a furnace, or whatever right now. Many more are also capable, but don't know it yet. So rather than demonizing something for not being able to basically time travel, environmentalists should promote green ideas in order to accelerate their adoption.