You can't move that single 480i mode in your list because it's the mode in the EDID. To fix your issues you need to...
Firstly, follow one of Calamity's setup guides, step by step, right to the end. That will install more modelines. For Radeon HD 5000 series cards or newer, this is the guide -
https://geedorah.com/eiusdemmodi/forum/viewtopic.php?id=301Next is horizontal timings (positioning). Below is the arcade_15 range line from the monitor.ini file. In bold are the values for HFrontPorch, HSyncPulse, HBackPorch respectively, and you can find them in ArcadeOSD when editing. Your values will be different, remember.
crt_range0 15625-16200, 49.50-65.00,
2.000, 4.700, 8.000, 0.064, 0.192, 1.024, 0, 0, 192, 288, 448, 576
You find these timings by editing modes in ArcadeOSD. Write down the numbers you settle on. Then either edit monitor.ini (just copy your existing preset and name it something different, and then overwrite your timings) and redo VMM and setting up your modelines. Or simply edit those timings into the mame.ini crt_range0 line. I prefer the latter, as i'm using a wider spread of modes than most people and it works better for that. And for specific machines or games you can create files like neogeo.ini or ffight.ini in the mame/ini directory, if needed, with the range line adjusted.
When you set up GroovyMAME, make sure to leave the infoscreens enabled. That means when you load a game you get a screen showing the game's native resolution, and the resolution switchres has picked. If you're using super resolutions, the vertical mode picked should be the same or slightly larger than the native res. 2560x240p is fine for CPS1 games at 384x224p, for example, and will leave 8 blank lines top and bottom - assuming you've set integer scaling for the vertical res, of course.
Now, once all that is done, there's your converter. Cheap S-video/composite converters will simply produce 480i no matter what the input is, and they tend not to take 480i input. Yours looks to be a higher quality model, i'll grant, but that site provides no specific information about signal formats. A good converter will output the same signal format as the input (so a RGBS input at 384x224@5997Hz comes out as exactly that, only as S-video PAL/NTSC). So, when a game loads at 240, does the image look nice and solid, of do horizontal lines still flicker? A good way to check this is to set your desktop to 320x240p and take a look at horizontal window borders. Then go back to 480i and see the difference, because it should be pretty marked. (Obviously the resolution is a lot lower, but the flicker of horizontal lines should be there at 480i and not at 240p)
If you give any of this a go, and are still confused, have a read of this guide:
https://www.aussiearcade.com/forum/arcade/m-a-m-e-emulation-projects-and-discussion/89704-a-guide-to-connecting-your-windows-pc-to-an-sd-crt-tv-pvm-or-arcade-monitor#post2194681