The boxy-nose is where things get tricky, as without a complete CFD simulation of the complete relevant structure - and the nose, being up front, affects the whole car - we have no idea how it works. Plenty of thoughts and ideas (and many of them negative), but no actual proof. My theory? The hollow underside of the nose, together with the low-reaching "skirts", acts as a diffuser - and a quick-and-dirty CFD run on F1Technical confirmed that this increases both drag and downforce - but again, no real idea.
Engine-wise? The Renault might have a negligible deficiency, or a negligible advantage - but not anywhere near the scale of last year's 20-30BHP deficit. As for RBR-vs-RF1, the engines will, obviously, be completely identical units...