Why does my apple ID enable all phones to see each other's stuff? That's idiotic.

Check out Family Sharing.

Family Sharing - Apple Support

Set up Family Sharing - Apple Support

http://www.apple.com/icloud/family-sharing/

Regardless of your opinion about how it should work, the simple reality that if you share a single AppleID for your iCloud, iMessage and FaceTime accounts then yes, each device will share all content for those accounts. The ONLY solution is to use separate AppleIDs for those services.

You can still use a single AppleID for the iTunes and App stores, or you can set up family sharing for those things.

On each device:

make sure that messages aren't being forwarded by each phone:

>settings >messages >Text Message Forwarding

make sure that content isn't automatically downloaded to devices:

>settings >iTunes & App Store >Automatic Downloads

to stop contacts being shared, you need to be sure that you aren't synching to the iCloud (even if the iCloud Drive option is switched off, contacts will sync if they are switched on:

>settings >mail, contacts, calendars >iCloud [then switch the relevant options off

HTH

In that section I can see the following options, in this order:

iMessgae

Show Contact Photos

Text Message Forwarding

Send Receipts

Send as SMS

Send & Receive (look at and change these setting as well)

MMS Messaging

Show Subject Field

Character Count

Blocked

Keep Messages

Filter Unknown Senders

Expire

Raise to Listen

Expire

Which of these can't you see?

Rgds.

Okay - text message forwarding is something that I believe is activated when you switch on iMessage. IF it isn't showing on your phone, then I would suspect your messages aren't being forwarded. However, if someone has got iMessage switched on on their phone then you need to check the "Send & Receive" and "Text Message Forwarding" on their phone to be certain they aren't receiving your messages or that their messages aren't being sent onwards to you.

Turning off all the sync settings in iCloud should make sure that none of your devices are affected by updates done on another phone.

IiMessage, iCloud and FaceTime are each independent services from Apple that any one person may choose to use on multiple devices that they own or control. As such, they are effectively subscription services tied to an AppleID. If you use the same AppleID for those services on multiple devices, each is in fact using the exact same subscription service account and will share the same content. If you dont want that then everyone needs to get a separate AppleID for those free subscription services from Apple. It's very simple really, if you don't want to share those services then each person needs to sign up for them Using their own AppleID.

if you do that you will NEVER have any issues with sharing content - before, during or after any updates, new devices, etc.

As I said you can continue to use a single AppleID for iTunes and App Store purchased content or you can use family sharing. Anybody can have multiple appleids and use different ones with different services.

Think of this ...

You can't share a phone number. I think we all see that as making sense. look up "party line" and see what used to be the norm in the telephone's early days.

In the past, an Apple ID did not "do" what it does today. Now it's more like a phone number and must be unique so it can isolate the various things it does.

the devices are linked to the account, yes. The devices are linked to each other... Only if you want them to be.

the telephone number analogy is terrible 🙂 (extension lines and different ringtones spring to mind lol)