I'm not "the expert", but I'll offer what help I can.
DVI Support: I am using the AmigaKit supplied video card with an HDMI connection, and have had no troubles so far.
I have a DVI to HDMI cable here that I'm using with another device, so worst case, you should be able to get going that way.
But the next answer might help faster..
Monitor Modes: When the OS is installed, the "maximum video mode" must be selected, so a reasonable mode had to be chosen for you.
I believe the easiest fix might be to go to prefs/screenmode, click on the "Monitors" tab, and make sure "Detect Settings automatically" is checked.
If everything goes as planned, that should adapt your settings to your monitor without any chance of making mistakes.
Well, that did not go well.
I did as you suggested above and clicked on the "Detect Settings Automatically" check box on the Monitors tab of the Preferences Screenmode program, which told me a reboot would be required to detect the monitor and change the monitor file. So I accepted all the requesters and clicked on "Reboot".
When it booted back up the screen looked like crap and all of the Radeon screen resolutions were gone and the only resolution that works now is "Bootup in VGA", or something like that.
I looked for the OS4.1.5 install CD/DVD, but one was not included in my shipment. So I can't even do a clean re-install to start over.
Just sent a message to AmigaKit asking if that was an oversight, or perhaps the OS4.1.5 install CD/DVD's are not back from the company that is mastering them yet.