1) Is Covid-19 equally as easily spread or deadly as the Spanish flu of 1918? More mild? Worse?
Easily, if not greater to spread, with the same carelessness taken then when you factor in the incubation period. Less deadly, but I hardly believe that all reported cases were because of the Spanish Flu itself and not secondary conditions directly relating to it, as we'll see with COVID-19 when medical facilities are closed, more plants are shut down, and entire cities are blocked off for essential shipments and goods due to quarantine measures to recross the line. It'll still pale in comparison.
2) Is the answer obvious or ambiguous?
Statistically, it's obvious. The ambiguous part are the 100s of unrelated deaths going unreported from secondary conditions, such as a heart attack or diabetic not getting insulin, because the entire medical community is overwhelmed in some places combating COVID-19.
3) Are comparisons even valid?
It is valid to make, to an extent, as those that didn't die from Spanish Flu died of other ailments as well because there were not enough attending. We could see that happen here, easily. How it'll correlate with what we know (and have) now about social distancing, washing hands and using hand sanitizer as well is what will keep us below the curve.
What doesn't help are idiots thinking that this isn't serious and traveling anyway—prime example is spring breakers in the US flocking to Florida where I believe 6 were already confirmed from 1 university alone while they all partied with 1000s.
So if we have less Darwinisms running around, COVID-19 will still be less than the Spanish Flu, but topple annual seasonal flu numbers nonetheless. If people continue, the pandemic will outgrow that, but not to 1918 levels.
I will safely put it between the 7th cholera pandemic (1961–present) at 570,000 and the modern plague if unlucky (1894–1903) at 10,000,000.
However, with it now effecting young to die, and many with underlying conditions. However, if this does reach impoverished nations on a wide scale, it can kill millions more in Africa as there is a 1.2 billion population and sickle cell—which would be very bad for a respiratory illness as it delivers oxygen to the body—occurs in 1 out of 365 births. Let's just say, 500,000,000 are affected by that in Africa and apply those numbers: 1.3 million births. All wiped out. There are also 12.7 million African Americans in the US, same number, 11500 could be at greater risk. Applying to just African population alone prone to have sickle cell and it's not improbable that COVID-19 directly effects more where we do reach the 20 million of 1918.
It's all up to finding a vaccine and dispersing it as quickly as we did for Swine Flu, which will be more of a logistical challenge as not many countries were effected then, while we're seeing COVID-19 in every, or almost every country now. More vaccines will also have to be created because of the incubation period too. I doubt it'll be a year to do that, I'm betting on 1.5 to 2 years.