Please know the need for a derailleur adjustment does not reflect badly on the bike or bike manf.
It's more the kind of thing that should be checked/adjusted as necessary when the bike was initially prepped for use. That would be the prep YOU were responsible for, unless you had a local shop set the bike up for you initially. At that point, it was them that was responsible. Every nut and bolt on that bike should be gone over when new. You, the buyer, are responsible for the final inspection and testing of a new bike.
As far as the battery connection, was the bike ever ridden with the battery not snapped in completely maybe? Whatever, if it is the battery connection that's not allowing the bike to power up, it's not something that requires anything special to clean up.
Please know I'm not here trying to beat you up. More just allowing for the fact there isn't a bike made that's 100% guaranteed to arrive in a box fully ready to be ridden. Just as the fact there isn't a bike made that's never going to need maintenance. Even a 2 week old bike. Actually, it's not that unusual for the cable to that derailleur to stretch a bit as it "breaks in" the first 50-100 miles of use.
Best of luck whichever way you decide.