Base level and Paragon level are different dude

level requirement only applies to Base level, not Paragon Level

Paragon Level are account wide, all your chars have their own paragon point by the paragon level you have,

Hardcore chars have own hard core paragon level

Softcore chars have own soft core paragon level
if you play new toon from lv 1.  you are lv 1/current paragon lv

you don't have to replay story campaign over and over for going higher difficulties

you won't receive experience point for paragon lv before your toon reach lv 60 max (in RoS is lv 70 max), after max base lv, you will continue earn Experience point for paragon lv

any difficulty monster lv will be scale by ur current base lv, higher lv you are, tougher monsters are,  but if u pick higher difficulty, that's even harder
i.e :  you play lv 43 toon on Expert difficulty, it will be Expert (43)
and for Hardcore, basically the same, just you have a different paragon than the softcore, you keep paragon lv if ur HC toon died, so ur new HC toons will still continue the paragon lv

Reduce lv requirement gears  (up to 30) will still useful for below down to lv 40 character if the gear is originally for lv 70