A hard seat - or perhaps more properly a "firm" seat - is generally better so that you can sit properly and not "sink" into the seat. I used to have a more comfort oriented seat on a hybrid bike - never liked it.

One reason why humans have muscle-skeletal issues is partly because we are upright. Gravity acts in only one direction, and compresses everything down unless we counter it with proper posture, exercise, and diet. Animals that walk on all fours distribute the weight of their internal organs, etc. across two points of contact, and the spine acts more like a suspension bridge to cradle your torso.

... but I'm not sure if this analogy applies properly to a biking position in which your shoulders are still higher (significantly so most of the time) than your hips: you're not truly on "all fours" like most animals. Also, you're maintaining constant or near constant tension in your arms for extended periods of time. I don't know of any sport - competitive or recreational - that advocates constant tension: I suspect that after a while, you'll really feel the effects. Maybe you're a lot younger than me...

Stiffening is not necessarily a sign of strengthening. You may not be feeling fatigue, but your body may also be compensating, and if you're young and in decent shape, your body can "absorb" a lot of abuse and not recognize the signs. A strong muscle is supple and flexible, rather than stiff.

Well, at the end of day, you'll have to come to your own conclusions of course, but I think some of us want to make sure you take care of yourself!