You should choose a default preview size that best approximates your monitor's resolution[1]. If you frequently want to zoom in, then rendering 1:1 on import might be a good idea. Be aware though, that even when the previews are already rendered 1:1, there still is some load time, because the files have to be fetched from your disk.

Considering your second point. Previews are always rendered in the background after import. You can see the current job on the top left. There will be one for import first, and afterwards another job will start for rendering the previews.

If you want to start a render job, some time later after import and maybe a program restart, select your photos then got to 'Library -> Previews' and select the desired size.

I would assume previews are discarded with their full size counterparts upon deletion. If not 'File > Optimize Catalog' should take care of this for sure, there is no reason to keep them if you deleted the photos from disk. There is also 'Library -> Previews -> Discard 1:1 Previews' if you want to be sure.

[1] Adobe says: 'Note that when using a 23" or larger monitor it to is best to set the Standard sized previews so that they are rendered at 2048 pixels', see here: http://forums.adobe.com/thread/358026