Your servent, whichever you use, obtains the files by sending an HTTP-format GET request. The parameters of that request include the range of bytes you want from the file. That is how resuming is possible; you simply request the range of bytes you don't already have, and append it to your (partial) file.

More complicated servents can and do request different ranges from different hosts simultaneously. Doing this shared a requirement with resuming; you must ascertain that the file being shared is the exact same file you already have a part of.

But there's nothing preventing multiple source downloads.

Since most Gnutella servents have bigger fish to fry, I prefer to just start two downloads of different material from different sources; they'll finish at the same time they would have using the other method, as I am narrowband (therefore using two sources guarantees that my bandwidth is always full). You may want to use five...