If I disable Messages in iCloud on MBP, delete all content, and then turn it back on, will it restore all convos and attachments?

To your original question - If I disable Messages in iCloud on MBP, delete all content, and then turn it back on, will it restore all convos and attachments?

No , the reason messages are synced only with iCloud , they are saved in devices Mac / iPhone , you will not see a message icon when Apple ID is signed in at www.icloud.com

The syncing happens when messages box is checked , the messages are only synced with iCloud , they are not literally seen at iCloud.com

To your second part of question - My actual question was: if I then turn Messages in iCloud back on again on my Mac, what happens then? Does everything stored on my iPhone/iCloud then get restored back onto my Mac?

When you turn back messages in Mac , messages will sync only when message box is checked in iPhone , yes those messages will again restored back essentially from iPhone and not from iCloud .

When you delete attachments from Mac Delete messages and conversations in Messages on Mac - Apple Support 

Set how long to keep messages

1. In the Messages app https://help.apple.com/assets/63B87598CE32D578D65A85BE/63B87599CE32D578D65A85C5/en_GB/ae8d596d44d76eddfb1ea9eb4772e949.png on your Mac, choose Messages > Settings, then click General.
2. Click the “Keep messages” pop-up menu, then choose an option.
3. Important: If you choose an option other than Forever, your conversations (including all attachments) are automatically removed after the specified time period elapses.

That file will remain in iCloud and/or on my iPhone when iPhone and Mac are signed in with same Apple ID and password on the same network , the conversations are shared between them as messages box is turned on , so deleting conversations with its attachment will be deleted from Mac , iCloud and synced device iPhone .

See the article Access your messages on all your Apple devices - Apple Support

You can turn off Messages in iCloud on all your devices or on your Mac only.

1. In the Messages app https://help.apple.com/assets/63B87598CE32D578D65A85BE/63B87599CE32D578D65A85C5/en_US/ae8d596d44d76eddfb1ea9eb4772e949.png on your Mac, choose Messages > Settings, then click iMessage.
2. Click Settings, then deselect Enable Messages in iCloud.
3. Choose one of the following:

• • Disable All: Turns off Messages in iCloud on all your devices. Messages are no longer stored in iCloud and are instead stored on each device.
  • Disable This Device: Turns off Messages in iCloud on your Mac only. Messages on your Mac are no longer stored in iCloud; on any other device where Messages in iCloud is turned on, messages continue to be stored in iCloud.

1) When you say "message box" are you referring to an individual message/conversation? For ex: If I open Messages app on iPhone and tap on a conversation with Jane, it will open the "message box" for me and Jane? 

Reply : No , in messages prefernces > settings the box of enable messags in icloud should be checked when you open messages app on iPhone , and tap a conversation with Jane , it will open the conversation with jane , not the message box .

2) When opening the conversation with Jane, will it only sync that one message? Or does doing that cause all conversations on my iPhone to then sync to my Mac? 

Reply : Conversations to a single message is attached , their could be conversations with diferent people and every conversation is confined to an individual message , when you open a conversation with jane it will open say you opened from mac , as you tapped on that message , but it doesn’t mean that conversation will also open from iphone , even the iphone / mac are signed in with same apple id and password on the same network , messages are synced ( this is two way syncing ) .

3) If I delete all content on Mac and then re-enable Messages in iCloud, nothing will happen, because there's currently no messages stored on my Mac? And I would need to actually open a conversation first in order for any syncing to take place? 

Reply : When you delete all content from mac , it mean all messages plus the conversation will also be deleted , and if you renable messages in icloud , you can’t open that conversation in fact it was attached with it and is deleted forever .

4) Lastly, just to confirm what you said: If I re-enable Messages in iCloud on Mac (and don't touch anything else), then open Messages on my iPhone and tap on a conversation, all messages on my iPhone will then sync to Mac (effectively restoring everything back to my Mac) and all iPhone messages will be recovered and accessible once again on Mac?

Reply : No , if you re - enable messages in icloud on mac , and tap a conversation it will simply open , but it doesn't mean all your messages will sync to open .

Syncing of messages is active when icloud messages is checked , the same apple id and password is signed in between the devices , in fact all messages are already synced , tapping a conversation doesn't sync all mesaages .