Has the composite to HDMI converter better quality than composite to SCART to HDMI?

The yellow-RCA is Composite (1ch), which is not as good as S-video (2channel - both are analog). You may need to replace the s-video cable(s) (most are DIN but SCART and DVI also support s-video), then convert to HDMI if necessary and follow up here as I'd be curious to know if you can see the difference.

If you're in the US then you probably have an NTSC system whereas PAL units were sold in the EU. Their associated SCART had different pinouts. The power is because your adapter basically has a little computer (IC) in there, and there's no 5v power from RCA. The distance of HDMI is actually shorter than analog but it has no loss (or less loss, I know some people recognize interference in longer hdmi runs but for the most part it either works or doesn't).