That Bentley & Skinner tiara is a beauty. But I think that in case they decide to purchase a new tiara for Kate they will be extremely secretive. Remember that nothing much was ever known about the origin of Sarah's tiara. Also Mette-Marit and Mary received antique tiaras for their wedding, but nothing became public about the origin, price, vendor etc. I just can't imagine that the BRF will buy a tiara with the price tag all over the www. But surely one or another jewellery companies would have a nice anonymous tiara in their safe...
Re. the Maria Feodorovna sapphire bandeau: I'm not fond of wedding tiaras with coloured stones, but maybe the sapphire can be interchanged with a nice diamond element? It would be great if Catherine received a versatile tiara.
Imo it would also be an option that Catherine received the sapphire bandeau as her permanent tiara, but borrows the fringe tiara, which has a certain tradition as wedding tiara, for her wedding.
All in all there are so many possibilities! When Victoria married I was 95% sure that it would be the cameo tiara. But with Catherine there is no tiara which I would give more than 30% probability.