Hi Pha,
Assumed as you designed required data flow.
1. Safe side better to lock users and reload full load data or init with data into bw side.
later schedule V3 job at ECC LBWE, job control.
now unlock users and continue with delta loads at bw side.
2. At least you may need minimum downtime, other wise data loads may leads duplication.
Discuss with client/users and explain about handling inventory data loads to convenience them.
As of now they won't accept to do this activity. you can do this activity at next week at non business hours or weekends wit client approvals.
3. Those are mandate above bw 7.0 version. so please install required data flow from BI content and start data loads.
Most of the helpful docs are already shared sander, please go thru them and be familiar with required procedure to handle inventory data loads.
Thanks