@PlanetJupiter I've been reviewing the api docs and playing in my sandbox env for both systems and I don't see anything Celigo can't do here.
On your first need, I'm not sure what you mean by "see" custom fields. We would just map to them. The only thing we need to know is the custom field id. I got this setup in roughly 15min and you can see my flow was successful and that the custom field I made had the hardcoded message posted into it.
On your second need, I test this out as well and it is totally doable. Also see attached screenshots of the message in fresh service matching the message in NetSuite.
Of course this would have to be built out more to handle duplications and additional field mappings, but again I see no reason why we couldn't support this. If you want to reach out, then I can check what account rep you are working with and get assigned to it to further help. Of course I can't build all these flows because then I would just be doing the implementation for you 🙂. I've also attached the flow zip so you can install this in your celigo trial account and play around.