Yes their csr leave a lot to be desired. I spent 15 minutes on the phone today with a csr insisting that a hold that had been placed on a PAYMENT which lowered my available credit was a PURCHASE! After insisting I made no such purchase she finally found out it was a hold on funds I had paid, (eg I have a credit balance on my account but my available credit is lowered - until the check that was sent by my BANK because they won't accept ach transfer payments cleared).
So
1 No I couldn't get them to eliminate or lower the cash advance
2 If you prepay you lower your credit limit
3 You cannot do an online payment (maybe unless you have a barclays account) so - Fidelity has to send a check - which takes a week for them to receive, then barclays puts a hold on it for 8 BUSINESS DAYS which lowers your available credit by the amount you have paid! I guess the only way to cut off a few days would be for me to mail a check myself? maybe decreasing by a day or two.
Conclusion: Good for the bonus only I think. No way to increase credit limit (which is low compared to others) by paying in advance and this, in fact, LOWERS your credit limit.
Will cancel shortly after bonus hits - other cc have better csr, will allow me to set cash advance to 0, post payment to my account within a day or two, etc.