Some people say 'further' and some 'farther'. Some people say 'furthest' and others use 'farthest'. Both are correct; it might be a dialectual thing.
Farther/further = more distance (to travel, etc). Furtherst/farthest = самый больший . The furthest/farthest I have to swim is 200m to the other side of the lake.
Meeting no further difficulties. Yes, it is the only variant I have heard.
Perhaps "further' is used for 'extra/more' etc. and 'farther' gets confused with distance, because it has the word 'far' in it. Perhaps English speakers historically have been confused. There are variations in English, because of the different migrations and dialects of the Anglo Saxons and all the other ethnic groups in those early days.