Are the MetaMask account addresses fixed once and for all for the same seed

If you were using the Google Chrome browser in Windows 10, you should be able to find the location of the Vault by going to this folder location (you need to be able to see the hidden files):

C:\Users\USER_NAME\AppData\Local\Google\Chrome\User Data\Default\Local Extension Settings\nkbihfbeogaeaoehlefnkodbefgpgknn

In that folder you'll see a file called 000003 (or something like that), open that file with a text editor or code editor software like Atom (https://atom.io/).

In the text editor search for the word "vault" and copy this part that looks like the one pasted below:

{"vault":"{\"data\":\"wwpXXtFCqZkYsWfeEwItZjJ0Cc7mRVjG47Dqh+ztL1PiCG6Izhg+zG0mM+H2ykyjz3X0RNhAE6IVsWFZamcZ47B4sVi4SvUxrMhARm5L3yHPxr3UsyGrOXmthyVMgEGmjwlmnFCNd2nMZ2o8/sRMra8FupurqevnBv57FiYpEEs7gPpFHv6587aL44MmKD8Snv4JLFqiqmlK82Waq5F+Iv9mw2sFVAL9mgZBSgFgbWdB3TsKVB2k\",\"iv\":\"rkUQlNcGTxBE0My7a/bCXw==\",\"salt\":\"HcKyNfGzaRALRQ0DlKgcIe5Uk30iI/M//oG6w8vX8Nk=\"}"}

Remove the two parts in bold. Now use the Search and Replace feature of the editor to change all the \" to "

You would now have something like this:

{"data":"wwpXXtFCqZkYsWfeEwItZjJ0Cc7mRVjG47Dqh+ztL1PiCG6Izhg+zG0mM+H2ykyjz3X0RNhAE6IVsWFZamcZ47B4sVi4SvUxrMhARm5L3yHPxr3UsyGrOXmthyVMgEGmjwlmnFCNd2nMZ2o8/sRMra8FupurqevnBv57FiYpEEs7gPpFHv6587aL44MmKD8Snv4JLFqiqmlK82Waq5F+Iv9mw2sFVAL9mgZBSgFgbWdB3TsKVB2k","iv":"rkUQlNcGTxBE0My7a/bCXw==","salt":"HcKyNfGzaRALRQ0DlKgcIe5Uk30iI/M//oG6w8vX8Nk="}

You can now go to the Vault Decryptor https://metamask.github.io/vault-decryptor/ and paste that in the textfield there, enter the password you set for the extension when you created your MetaMask wallet. You should be able to see your seed phrase and any manually imported private key below if you click the Decrypt button.

You can get your accounts back by clicking "Create Account" (once per each account).

The accounts are there, however they are not shown until you add them.

If the accounts that show up do not match what you are expecting, then you are using an incorrect seed.