Then why did you start a thread asking about hill climbing?

I would never use crankarms that long on a fixed gear bike. When you lean the bike into a corner while your legs are spinning (because they can't stop spinning on a fixie), you will strike the pedal on the ground and you will crash hard. I would never use 190s on anything, but you may have giraffe legs for all I know. For me, it's 175mm on singlespeed, but on a fixie I like 165s.

You should read Sheldon Brown's articles about singlespeed and fixed gear conversions very carefully, at least three or four times. He covers everything you will need to know much better than I can here. See links below.

Let's get some terminology straight: A hub that allows you to coast without pedaling is called a freewheel. A coaster hub is what 's on paperboy-style Schwinns, where you push back on the pedal to activate a rear brake.

Neither a fixed hub or a freewheel hub is "better." They are both fun, in different ways. IMO, a singlespeed gives you the most flexibility in gear choice, because you only have to consider how hard the gear is for climbing, and you can ignore it for descending. On a fixie, you need a gear that is low enough to climb with but high enough to allow you to pedal on the descent without losing control.

Yes, it's manageable to climb steep and long hills on a single speed bike, provided the gear is low enough and/or you are strong enough. But you said you don't have any of those hills where you ride.

Since you don't have any extended or steep climbs, try a 42 X 16 and adjust from there.

Sure, maybe. Put your geared bike into one gear and ride it without shifting for a few days. You'll eventually figure out a combination that works.

Using your leg strength to slow the rear wheel down is something you need to experience to understand. Going downhill on a fixed gear really isn't about controlling the speed of the wheel through the pedals; it's more about maintaining control of your pedals at the speed the rear wheel is pushing them.

If you have chronic knee issues, I think fixed or singlespeed would be the opposite of a good idea.

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READ THESE BEFORE DOING ANYTHING ELSE:

Singlespeed conversions: https://www.sheldonbrown.com/singlespeed.html

Fixed gear conversions: https://www.sheldonbrown.com/fixed-conversion.html

For more "which gear do I want" advice, read the responses to the other threads you've started on that topic:

If you Could Only Pick one Gear...
What Sort of Gearing Works Best for your Needs?