Hi
well firstly let me address the greens ... its overblown and 90% myth. Take the bull by the horns and self test ... even if you just do it to learn, self test ... then you know. Self test and document, then you learn.
This is the basis of science and the modern world around us: you don't just believe what someone says, you know by verifying what the theory is, and you verify by testing.
Next I'm a cross country skiier (I'm an Australian but back in Finland now waiting for the season to get going, so far its only slush), but I have never found INR to be an issue with my skiing. Nor indeed has any study shown that INR under 3 leads to problems.
My advice to you is to keep the INR about 2.5 (of course it'll be higher or lower occasionally) and test weekly so that you know.
The issues with higher INR are bleeds, and high INR does not cause bleeds it just allows them to continue. Most of the data you'll read is about old people. Old people have less of their elastin still being elastic and so what would be a stretch of the blood vessels on impact for a younger person is a rip on impact and a bleed. This is very well known in trauma work where they even have a rating system to assess the survivability of an impact and guess what ... AGE is the biggest factor in determining the survivability of an impact.
http://www.trauma.org/archive/scores/iss.html
I also ride motorcycles (as well as ski) and take safety very seriously, and the AIS score of impacts and the ISS (Injury Severety Score) is used to determine if you are likely to survivie, and guess what, for any impact it is worse as you get older.
http://www.pitt.edu/~epi2670/severity/severity.pdf
The primary factors of interest include age and co-morbidity. Despite similar injury severity scores, the prognosis for recovery may differ by significantly by age, with older individuals faring worse than younger individuals
So unless your daughter also has a vascular disease then I'd say she'll be fine (given that I usually am) ... but you know ... wear a helmet right?
I've not hit the road (fell off) for some time now on my motorbike (and I like to keep it that way) but I have gone whack on the bicycle (and skiing) and so far being on warfarin AND being over 50 hasn't killed me.
My advice is that the primary worry you have is the unknown ... you've been built up to worry about warfarin and its impacts, largely because people who just don't know tell you things which they hope will guide you ... but the reality is that if you're keeping your INR under 3 and better yet under 2.4 it'll be A-OK
I disencourage you to have your INR lower than 2 as from what I understand it leads to increased risk of clogging the valve with thrombosis formation. That's reversible these days non surgically, but I like to manage myself to avoid hospitals at all (yes I know ... why ride a motorbike ... ;-)