Question 1 : Yes , you can transfer your Tessa only ISA to any other provider who accepts transfers in. DON'T close the account just get the new provider to arrange the transfer from Nationwide.
Q2 ; Are you sure that Halifax will not let you top up your fixed 1 yr bond ? Certainly worth a phone call - I was allowed to top up my fixed 2 year bond with Principality; even if they won't let you add new money to the fixed bond they may let you put the £1500 into another ISA product with them which is allowed under HMCE regulations. You don't have to wait for your 50th birthday; as long as you are 50 before 5th April 2010 you can top up now.
edit - there is a notice on the Halifax website
IMPORTANT - existing customers If you are an existing fixed rate
ISA Saver customer, aged 50 or above on or before 5 April 2010, then you can make a one-off extra payment into your account
(only if contributed in current tax year) before 6 April 2010 to take advantage of the new
ISA limit of £5,100.
Just call 08457 26 36 46 or pop into branch