"Hand in hand" is idiomatic (in both English and Swedish, apparently" for "holding hands". If I was expressing the idea in English I'd probably say "They came along, hand in hand", but I suspect "walking" is in there because it's gående, not åkande. ("De kom åkande hand i hand" sounds dangerous, if nothing else).
There has been an increasing number of complaints regarding missing word tiles since the new design was introduced. Unfortunately, there's not much I can do about it - but there are some things you can try. First, try zooming out in your browser, this will usually cause the missing tile(s) to reappear, and you can then zoom back in after answering the question. If that doesn't help, I encourage you to submit a big report here: https://support.duolingo.com/hc/en-us/requests/new