Yes.
You can use all VOCALOID3, VOCALOID4, VOCALOID5, and VOCALOID6 voicebanks with VOCALOID7 Editor without any special steps or settings.

Project files (VSQX and VPR) created with VOCALOID3, VOCALOID4, VOCALOID5, and VOCALOID6 can also be loaded.

Yes.
A free 31-day trial version of VOCALOID7 Editor is available.

https://www.vocaloid.com/en/vocaloid7/trial/

Installing the trial version installs both VOCALOID7 Editor and the included voicebank “asa”. You can use both the standalone and plug-in versions and start creating vocals right away.

You can also install VOCALOID7 Editor without uninstalling the older version you currently use. Please give the new VOCALOID7 Editor a try.

Yes.
You can use VOCALOID7 Editor on its own without any other music production software.

The full version of VOCALOID7 Editor (not the upgrade version) also includes Cubase AI, a digital audio workstation (DAW). With Cubase AI, you can enjoy more advanced music production, such as creating backing tracks.

Please see here for more information about Cubase AI.
https://www.steinberg.net/cubase/ai/

Please download it here.
https://www.vocaloid.com/en/vocaloid7/liteeditor/

VOCALOID7 Editor Lite is a feature-limited version of the Editor.
You can use it if your purchased voicebank for VOCALOID3, VOCALOID4, VOCALOID5, VOCALOID6, or VOCALOID7 has been activated.

It can only be used in standalone mode and supports a maximum of 2 tracks in total, combining VOCALOID tracks and Audio tracks.
It provides the same sound quality and editing features as the full version of VOCALOID7 Editor.