Workflow Leading Practices - Input Data to Parent Entities | Forms?

OneStream allows parent adjustments (as you've mentioned) however you'll need to choose between doing those adjustments via form or journal. This doesn't apply to base entities as you can use both forms and journals on them. You can change the account adjustment type via the grid view, use the metadata builder or extract an xml, carefully modify it and reload it.

Using CSV v Excel for forms... Think about what file type will be easiest for your users to understand and complete. The import to OneStream is identical in both cases. In my experience, the Excel route is far more common than CSV. There's a chapter in the Design & Reference Guide called Loading Form Data that walks through the templates found in the Sample Templates zip file on Solution Exchange as well as instructions on how to create a Form CSV template.