Hmm, well Methods is certainly much harder than GCSE, (are you sure you're not referring to Studies?) if you really would 'walk it' as you say, then there is nothing really to stop you doing higher, ultimately HL is not THAT difficult. Methods would still be perfectly acceptable for PPE tho, it certainly wouldn't automatically exclude you by means. As regards Economics well, I suppose it depends on you, it's one of those subjects in which people tend to do very well or very badly. A serious Oxford PPE candidate shouldn't find it too difficult by any strecth of the imagination though.