This is planned, see “Bloom filters”, it’s a big 0.9 feature (might make 0.8 but increasingly unlikely), it should improve performance considerably. Because of recent changes ensuring that your local requests are not stored in your datastore, we can do this safely for Friends, and even for Strangers (opennet peers) if we contact them often enough.
The Keepalive plugin (unofficial so far) allows you to make sure that particular files remain fetchable. This needs further improvements and it needs to be made an official plugin, but it solves most of the above. For security’s sake, we generally want the original uploader and the people running Keepalive for the file to be different. It is planned to improve Keepalive, make it official, and integrate it into the web interface.
Perhaps, but right now donations are barely enough to pay for Matthew, and without him the project would be in serious trouble. Even as it is Matthew is seriously underpaid. The ideal outcome would be for people to volunteer to work on 3rd-party apps (this is already happening). Where donations are concerned, we really can’t afford to divert them from the core project with the current financial situation.
It is not necessary for a user interface to be “standalone” for it to be easy to use. The most urgent priority has to be improving the existing web interface: Make it easier to use, make it easier to do what you want to do (chat, filesharing etc). Currently, xor is working on improving the WebOfTrust backend so that Sone (our microblogging tool), FlogHelper (a site insert tool), Freemail etc will work better. Freemail v0.2 will be included in the next build and works much better than Freemail v0.1. We have a student working on filesharing, see the top voted poll. Meanwhile we have _mak working on redesigning the web interface to make it easier to use and more attractive/modern.
Yes, this will be a major priority in the near future.