Go to Manage | Database | Relationships and find the relationship used for this portal.
Double click the relationship line and clear the "Allow creation of records..." option. When you use this option, the "add" row in the portal is always at the bottom.
In either the relationship or in portal set up, specify a sort order that would put a newly created record at the top of the portal. Sorting by Timestamp or serial number field in descending order are two possible sort orders you might use.
Put a button on your layout to start a new record in the portal. Give it a script similar to this:
Set variable [$ID ; Yourtable::PrimaryKey ]
Go to Layout [PortalTable]
Set Field [Portaltable::ForeignKey ; $ID]
Go To Layout [original layout]
Clicking this button should cause a new, blank record to appear in the top of your portal.
Second question, you can use custom access privileges to deny a user edit access to a table unless the record has their account name in the account name field. See "Editing record access privileges" in FileMaker Help and check out this particular sub section: "Entering a formula for limiting access on a record-by-record basis" for a more on how to set this up.