We want to start over with Manager, so we have a few questions

you can set a starting balance for the suppliers.

if all these business is carried out from a single company name, you do not need to create separate business in Manager.
you can track the cash flow using separate cash accounts.
also you can use separate inventory locations for all the different business you do.
you can check the guides for these.

read this topic.

yes add them as an inventory. follow the same step as suggested in the above attached topic. or else enable and use production orders. if the packaging is a fixed quantity, you can use inventory kits under settings.

use credit notes. in case they do not want money back, just leave the value as blank.

not at present. you can use the Clone option though if most things in an invoice are same.

read this topic.

you can backup you business manually. also you can sync this to your google drive, etc.

the solutions provided are in general. you can find detailed guides for mostly everything in the guides section. also, you can search the forum for specific answers as most of your questions have already been discussed in detail by other users.