So you are forcing the use of Stealth Rock in the analogy to find some possible situation for Genesect to not be countered by Rotom-H. For one, this isn't Gen IV and SR is becoming less necessary to have up first turn, if at all. That is why pokemon like Volcarona are used now. Also Rotom can heal himself, and I also find it silly how you consider Genosect being forced out with U-turn, doing next to no damage to Rotom to be "momentum". Genosect either U-turns out (if he even carries U-turn, or he switches, because he cannot threaten Rotom-H at all. For a whole host of reasons (Burn, Para, Overheat, Pain Split)) meaning that Rotom-H is by defenition, countering Genosect.
With spinner support on your team, which is common, Genosect won't even be able to rack up damage on Rotom-H, while all it's attacks get resisted.