For the Strings sound, you can adjust the "Attack" to every sound you like. For Strings, some have the Attack set to Knob 1. So turning the Knob should change the Attack to either slow, which makes it fade in. Or fast, to make it like that Nightwish song.
However, for a more permanent change, you can go to Tone Adjust on the main page, and adjust the "F/A EG Attack Time" parameter (shortened "F/A EG A" on the screen, it's number 5), and then save it when you're satisfied.
Even though I haven't memorized what every string sound on the keyboard sounds like, I'm pretty sure that there is a preset with a quick attack as well.
About the Orchestra sound, the M50 does have plenty of Orchestral sounds in Combi mode --> 05 Orchestral. Among my favourites are "Orchestral Passage", "Concert in the Castle" and "Renaissance Ensemble".
What I usually do for fat orchestral sounds is that I use Orchestral Passage, and add a Timpani layer at the lower octaves, gives you quite a fat sound

If you do that, and perhaps add some more lower-register strings or some other type of bass-ish sound, you will have a very similar effect to that Roland