Max,
First, to answer your questions...
1. When you click the "Map DVB Services to Channels" button, all channels that have registered service and channel names get sent to your channel list. And yes, it should happen instantly.
2. The EPG depends on which service you're using. After the channels are in your channel list, go to "Configuration" -> "Channel/EPG" -> "EPG Grabber". Depending on what TV service provider you use, you will either be using an XMLTV source, or EIT or other "over-the-air" source. Set up the grabber on this tab to best match what you use. Once you click "Save configuration", you can go to the "Channels" tab and individually set each channel's "EPG Grab source". Click "Save changes" and you're done. It's a pain if you've got 739 channels, but that's how it works. Oh, and just a warning: EIT doesn't work at all for American ATSC sources.
Now, if you're not getting channels into your list, you might be having the same problem I am. Just to confirm that, could you answer a couple questions?
1. If you go to the "Services" tab, what do you see? Are any channels automatically named, or are they all listed as "Unmapped"?
2. Which version of TV Headend are you using?
3. What broadcast service protocol are you connecting to? QAM? ATSC? DVB-T, DVB-C? You have a lot of channels, so looks like cable or satellite.
I'm connecting on version 3.5.204, and my tuner hardware (Hauppauge WinTV PVR-1950) registers exactly the same as yours ("Samsung S5H1411 QAM/8VSB Frontend"). I'm connecting to ATSC antenna broadcast.
The problem I'm having is that after scanning in services, all service names are blank, and channel names are "Unmapped". In the 2.xx versions and 3.0-3.2, I could get most services to show up, but there were always some that were blank/Unmapped. But in 3.5, I've never had service names come up correctly. In order to get channels in my list, I have to give each channel a dummy name (I usually just number them consecutively: 1, 2, 3, etc.), then watch each one until I can figure out which channel I'm watching and give the channel its real name. Once you give a channel a name, it should automatically be added to your channel list. Being on an antenna, I only have about 50 services to watch through, so it's annoying, but not too bad. But if I were using cable with hundreds of services to go through and this happened, this would be an incredibly daunting task!