I note the warning about overwriting the preset "Warning : Preset number selected here is overwritten!" but I presume this can be reset back to a normal preset function if I decide not to use RokuRSS?
No, this cannot reset back to the previous preset. But, you can overwrite the preset manually. RokuRSS does not change the function of preset area. This software just uses the title area of the preset to display RSS.
Does it parse xml urls?
Yes. RokuRSS parses the RSS contents of xml urls.
Will Soundbridge retain the RSS settings so that RSS will display automatically and does it need Adobe AIR runtime to be running at the same time to display RSS?
Yes, RokuRSS has to be running while displaying RSS.
If you enter the RSS url, RokuRSS parses the RSS, and sets the item of the RSS contents in the preset of Soundbridge at specified intervals.