@Premm, most of the questions you post are covered by the Guides. In this case, see:
https://forum.manager.io/t/setting-up-starting-balances-for-suppliers/7227
I am not certain I fully understand your inventory situation. But after you have entered the outstanding purchase invoices, which are how inventory levels are normally adjusted, check your Inventory Quantity Movement report for your start date. If it is not correct, adjust your inventory opening balances accordingly.