Funds don't expire if you have an active contract -- they roll over into a new plan. So if you don't use your entire funding amount in a year (and the first year it's a bit more than a year b/c you get the rest of the month you opened the account plus 12 months) and you sign a new contract, your old funds will roll over into the new program year. If you don't sign a new contract, the contract allows you to request a 2 month extension. If the funds aren't spent and there is no active contract, they do indeed become expired/unusable.

You can add funds at any time, and in any quantity. In fact, this is pretty important because someday you're going to want to zero out the account so you'll likely need to add some odd amount so that you can use the rest of the funds and end with a $0 balance (although you may be able to split the payment between UATP and another form of payment, but even there you need to be careful because there are penalties for using non-UATP funds for PP-priced tickets). The catch with adding funds during the contract is that they don't earn amenity points, and the funds aren't aggregated for GS purposes. In other words, if you open the account with $20K and then add $30K, you'll earn amenity points on only $20K, and you will not get a GS nomination.

The amenity points can be used to pay for things like club memberships, status buy-ups, wifi subscriptions, travel certificates, and the like. The standard offering is to get 25% of your initial investment as amenity points. Note that at $50K, you get a GS nomination but you don't get amenity points. The rationale behind this, as explained to me by UA, is that with GS you don't need the things the amenity points can buy you. I don't entirely agree with that, but that's the way it works. You do earn amenity points on investments past $50K. For perspective, a domestic FC travel certificate that books into PZ costs $2600 in amenity points.

I hope this helps. Please post with additional questions!