Hello - sorry for the delay in responding.
I selected "Manually manage music and videos" from the start (6 months ago) and now want to add some more music (drag and drop - no problem) but also want to delete a tune from the iPhone here and there. No way to do that?
You cannot delete music from your iPhone regardless if you manually manage music and videos or you sync. To delete a song from your iPhone when manually managing music and videos, with your iPhone connected to iTunes, open your iPhone in the iTunes source list to reveal the iTunes content and categories within. Select Music, locate the song, and manually delete the song from your iPhone.
And just playing around I switched to "Synch only checked songs and video" and I got a message something like "Are you sure you want to do this because everything will be erased from your iPhone before this synch". Did that mean that EVERYTHING wuyld be erased (applications, contacts, etc) or just the current stuff in the "music" particion would be erased and replaced by what I had checked?
It should be just music and video. You can manually manage music and videos, or you can sync. You can't do both at the same time. If you are syncing and you switch to manually manage music and videos, all music and videos transferred to your iPhone via syncing will be erased from your iPhone first. If you are manually managing music and videos and switch to syncing, all music and video manually transferred to your iPhone will be erased first.
Since the iPhone does not support disk mode, I don't see much benefit to manually managing music and videos with the iPhone. A downside to manually managing music and videos is if you need or want to restore your iPhone with iTunes, you must remember which songs you manually transferred to your iPhone followed by manually transferring the songs to your iPhone after your iPhone is restored.
If you are syncing and you restore your iPhone from your iPhone's backup, all iTunes playlists and video selected to be transferred to your iPhone will be re-selected automatically, which must be followed by a sync to re-transfer your selected playlists and videos to your iPhone.
I sync and I choose selected playlists. I have a primary iTunes playlist I use for my iPhone along with a few other playlists for select music. To add additional songs to my iPhone, I add the songs to one of my playlists in iTunes that is selected to be transferred to my iPhone followed by a sync. To remove songs from my iPhone, I remove the songs from the selected playlist in iTunes followed by a sync. I also have sync only checked songs and videos selected. With this selected, I can remove a song from my iPhone without removing the song from the selected playlist in iTunes by un-checking the song in the playlist in iTunes followed by a sync.
IMO, syncing provides more flexibility and options, and if I ever need or want to restore my iPhone with iTunes, all I need to do is make sure the playlists are selected under the Music tab for my iPhone sync preferences that I want transferred to my iPhone followed by a sync without having to remember which songs I had transferred to my iPhone afterwards, which is required when manually managing music and videos.