You just need 1-hit kill on ER, but wait 5 seconds on Nightmare and Ultimate before going for the kill. This is the minimum time requirement for the run to NOT be flagged as suspicious. Very Hard (the easiest difficulty) has no such requirement, just go in for the kill.
No, but you can use the renaming of the com.klab.bleach trick (google "Bleach Brave Souls rerolling") to save you the trouble of the 1.8gb+ of data download. You have to sign-in separately when switching from playstore to mod apks, they are treated as different packages.
You can install <Account Linking> and <Mod> apk over one another though. If you have more than 2 accounts, you can switch accounts by patching one over another.