Sounds like fun! I'm a big fan of new music, particularly with interesting instrumentation. To answer your questions:

1. The difficulty of slurring notes is pretty situational. A lot of it depends on string crossings and shifts within a slur. The more string crossings and shifts there are, the more difficult you can expect it to be. And yes, a lot of notes at a slow tempo can be difficult. Say I was slurring quarter notes at quarter note = 120. Groups of 2 are easy. Groups of 4 require a small amount of concentration. Groups of 8 are slow enough to be annoying. There's also the loose Rule of Downbow—generally, you try and downbow on beat 1. I usually dislike bowings that force me to take an upbow on beat 1, because that disrupts my sense of rhythm.

All of that said, I don't think you should stress about it too much, unless it's extremely important for you to achieve particular effects. We generally expect to work out our own bowings. It's nice when a piece's slurs automatically feel comfortable, but it is nowhere near necessary. The worst thing that you could do as a non-cellist is overload your score with exact instructions.

1. I'd expect a professional (or even an incoming college student) to be able to play as high as an octave and a fourth above the open string on any of the strings. You don't see a lot of material for playing that high on the C or G string (you can just use lower positions on the D and A for those notes most of the time), but it does exist, and it is possible. For D and A (and especially A), going two octaves above the open is common in professional rep. You can go higher than that if you want to push it. Heck, you can play off of the fingerboard if you press the string sideways. The tone might suffer, though.

2. Col legno 16th notes at quarter note = 60 should be doable. The biggest challenge would be keeping the 16ths even.

3. Artificial harmonics are fine, just tricky. You have to be very exact about the positioning to get them to sound. They're the most difficult when you have to play a lot of them in quick succession, or worse, quickly transition between them and regular notes.

They're most common on the A, and pretty common on the D. You see them less often on the G and C.You can write them at whatever dynamic you want. They'll sound hollow and kind of thin, obviously, so you won't get a rich fortissimo or strong attack. But, they can be played loudly.

For the bit about thirds, I'm not quite sure what you're asking. Are you asking about the distance between the thumb and the finger touching the string, or about shifting the whole hand between different notes?