1) Cash will
ALWAYS exist, as long as there are bribes to be made.
2) How in the world are debit cards (seems to be what Valarian is contemplating) going to work? How long will it take for a debit to appear in your account? If you maintain the balance on the card, that will involve massive, easy fraud. Unless you use a Cymbeline chip...
3) I don't think
anybody would exchange money beyond a certain J-distance. Your info would just be too far out of date to be useful for exchange rates. Ancient peoples carried their trade monies in cold, hard cash for that reason.
4) Lanthanum would be valuable because it is the prime component of jump grids. And, it's just rare enough to make it valuable.
5) What would make a good specie? Almost anything we use now: gold, silver, various gems. All these things have value in a high-tech society (computer connections, chips, lasers, small displays, etc.) and have their standard "shinyness" factor on barbarian worlds.
Also, certain spices might be valuable. They are foodstuffs, and probably have very special characteristics, can be transported in a dried/cake form (so they don't spoil), and can be measured in small quantities to make change.
6) TAS would be the closest thing to an Imperium-wide bank, I think (at least, OTU). A TAS membership would mean you are good for a certain amount of
credit (which
would be on your TAS membership card).
This is the way banks worked (for those who had enough money to need a bank!) 200 years ago. You would deposit your money with one bank. Then, you would carry a letter of credit with you, good for a certain amount of cash. Once you deposited that credit note at a local bank, you could draw upon that credit until exhausted, or until you left. If you didn't exhaust it, you could have the local bank write a new letter of credit for the amount remaining to take to the next destination. The local bank could then settle with the original bank for the amount used (the whole thing if they wrote a new letter of credit for you).
This provides the mechanism for Edmond Dantes' revenge on one person in
The Count of Monte Cristo. Well, that, and the fact that he could interfere in the news system to spook speculators.