This isn't a stellar answer because there's more information needed. But here's a start.
First, your roof system is built with trusses, so the forces acting on the wood behave a little differently than classic stick framing. (For instance, some members are in tension; others are in compression.)
In picture #1, the crack: it's not clear to me what function that stick is doing. If you could draw a sketch of the truss (with the approximate location of the crack and an indication of where walls are underneath), we could form a bit more of an opinion. (Off the top of my head, that crack doesn't look terrible in the context of a truss, but we need more information.)
Pic #2: if you're highlighting the long crack, that's unlikely to be anything other than checking. (If it was an inch/25mm deep, that would be a problem, but I don't see that in the picture.)
Pic #3: if you're only able to dig in a few mm before you hit solid wood (don't hesitate to be aggressive with a screwdriver or pick -- any wood you remove wasn't helping anyway), I wouldn't be concerned about this. Check the other side to see if it's any deeper. If you manage to excavate a considerable amount of wood, come back and tell us. More important is the question of how that started to rot. Is there a leak there or above it on the roof? Or has that been dealt with?
Regarding your weight on the roof... don't worry about it. Modern roofs are designed to carry considerable live/dead loads, and you're not going to get close to the limits.
edits to respond to comments (I ran out of space in the actual comment box...)
I'd like to reiterate that I'm neither an engineer, nor am I in your attic. So my opinion might be worthless. But I look at your additional pic of #1 and I see a truss member that's in compression. And it doesn't look like it's completely falling apart either. So I'd sleep well at night with that over my head. But if you're concerned, you could add another 2x4 either beside or under the existing one. (Essentially, you'd be giving it a splint -- called 'sistering' if you want to google it.)
Good to hear that the other issues aren't problematic.
And a quick aside: trusses are astonishingly flimsy when they arrive at the jobsite. Carpenters look at each other as if to say, "We're gonna put that crap up?!?" The wood they use never seems to be particularly nice. And yet, these things work well. Partly because they are overbuilt, and partly because they are so redundant when put together in a system.
So, long story short, I'd suggest you not worry.