I agree on how well-written the game is. There are no magic money bags for the MC, he has to rely on his built relationships to help Rosi without massive negative impact. And there is a wonderful variation how you can gain the help you need. All variants feel different, because the characters helping Edwin are different. You do not see this variations in many other eroges. That is simply a thumbs up for TD1900 and GIL3D!Yeah Kath no strings attached is probably the "safer" for Rosa and MC.
But with this plot point you can see how well the game is written : you must have good relationship with Felicia for her to accept the deal, with August too for him to help you, probably with Killian too (didn't test but I assume if you have low bromance he won't propose it?), and also with Kath (and have the appropriate perk). Otherwise you can't help Rosa without negative impact even if you wanted
As for the solution "Kath with no strings attached" is safer for Rosi and Edwin, I am not fully convinced. As we have seen, Kath is very competent at using situations to her advantage, e.g. her charity events for sniffing out potential Carnations. So having her "saving" Rosi might not be so well in the long run and depending how the relationship Kath-MC develops.