Isn't it nice to give us more information?
So you have a back issue, then I wouldn't even consider a drop bar, in fact you're not going to be able to be more aero so I wouldn't even pursue that whole idea.
You might want to look into a bar called the Moustache bar, this bar will allow you to move your hands in more positions while riding but it won't require a new set of brifters, maybe new cables if the ones you have don't reach.
Nitto makes the best moustache bars.
Keep in mind, if you go with narrower tires they have to be pumped up with more PSI then wider tires, more psi means more road shock being transmitted up your **** and up your osteoarthritic spine causing more discomfort and or pain. Also hybrid bikes, like the one you have, are the best option for you, the only other option is a recumbent bike but you would have to test ride one for awhile to see if it would really work for you or not, but recumbents are expensive, like in the $2,500 neighborhood. Comfort bikes are also suppose to be good for the back but they have a weird riding style but at least you can ride a bike.
If I were you I wouldn't be worrying about speed, I would be worrying about being more comfortable, but that's just me, but from what you said about your back and your previous road bike you don't want to go to drop bars and you don't want narrower tires, if anything you should be looking at a wider tire then what came on the bike so you could lower the psi more so the tire will absorb more road shock; but you do what you want, sometimes we have to learn the hard way.