Sue, I'm in much the same position as yourself in that I'm not a native speaker, but I can't resist singing in it because there are so many good Gaelic songs. In my opinion you're doing the right thing by trying to understand a song as well as you can before singing it. That way you'll probably emphasise the correct syllables, pause in suitable places, etc.. I wouldn't be put off by what your Dutch friend said: different well-respected native speakers won't necessarily sing the same song in the same way or pronounce words the same way. I wonder what she would think of Runrig, Capercaillie or (from what little I've heard) the Vatersay Boys. I doubt if there are many native Gaelic speakers who would vote to disband Runrig, even if they disapproved of Runrig's departure from tradition. I think most of them would be pleased that Runrig are helping to raise the profile of their language.
   Most native speakers seem supportive, such as Julie Fowlis, who checked my translation of an Irish song into Scottish Gaelic while selling CDs in the interval although her show was already running late and her band were going to have to drive through the night to their next gig.
   I can understand that some people might think there was no point in singing Gaelic songs to non-Gaelic-speaking audiences, but strangely this doesn't seem to be a problem in my experience. They seem to lap it up. (As recommended above, I try to give a good introduction). Sometimes I'm actually asked to sing a Gaelic song.
   Don't be put off if you are sometimes told you're doing this wrongly or doing that wrongly. It can be difficult to get two people to agree on what is "right" and "wrong". I recall hearing a recording of a duo singing a song about the Isle of Mull on Radio Scotland once, after which the presenter said something like "I thought they would have reached an agreement on the pronunciation of Fionnphort!"