Kagome and Hazō have discussed it. So far as anyone is aware, seals eventually failing is simply a property of the universe, much like ice melting when it gets hot. Some seals are as robust as they can get, others are primitive enough that there is room to improve their duration or other characteristics.
Infusing a blank turns it into a seal that is capable of being activated. Activating a seal sets up 'machinery' that can pull wisps of chakra from the environment in order to produce a particular effect. People have considered seals that actually store chakra for later use, but according to Kagome it's generally considered idiotic, and no seal that Hazō is aware of does that.
See below re: skywalker seals, which are currently (and permanently, by Tower fiat) the most valuable.
"Patent" is too strong a word; there's no protection against having the seal copied, it's simply that anyone who researches and sells your seal needs to pay you 50% royalties for a couple years.
Yes, Jiraiya set this up. The price and number of these would be difficult for the Tower to pay in cash, so they are adding the profits to your clan stipend: Each month you get 10% as cash and 90% as an addition to your non-monetary-benefits account.