Can I replace hydronic water boiler return pipes with pex?
Here in Italy pex is widely used for new heating system (both radiators and underfloor), just check the maximum allowed temperature is high enough (I'd say over 120°C) and you could use it for both flow and return. The story is different if your heating is steam.

The slope could be there because originally it was naturally-circulated hot water (hot water) system with no pump, only the (relatively low) specific gravity of hot water 'moviing' the system.

In my opinion, if your new boiler have a circulating pump, you could hide your pipes anywhere and replace them with PEX