Currently a new/updated software entry is published immediately.
There should be a 'preview' function.

Yes, and no.

Might be useful, but it's also UI clutter and rarely used I'd think. Since the submit form is quite lengthy, it's unlikely that users would fill it out but then discard it after a preview. (And more honestly, I'm worried about introducing potential XSS holes with a preview handler.)

The real bug here is probably the "Please proofread before saving" warning.

It should be something akin to "Please proofread. Albeit you can edit the project/release as often as you like at any time." - which would avoid any publication anxiety.