How to use mod: Factorissimo2 with Brave New World
Any ideas?

Making BNW and Factorissimo compatible with each other requires a lot of work, so I don't think this ever will happen.

I think of some basic idea :
1) placing one roboport and storage chest per surface would be allowed, since we can have one robot in our inventory, this would permit to be compatible with all mods that add surfaces. this wouldn't require so much work ...

In BNW 2.0 (currently in development, hope to release it within a week or two), you won't be allowed to carry bots at all.

Supporting Factorissimo is a bit down on my priority list. I know it's a popular mod, but making BNW and Factorissimo play well together is challenging. Placing roboports and adding robots is a bit of a hackish solution. I'd much rather see that bots could "enter" the buildings, but this might not be feasible.

i don't think bots are able to enter the buildings, but you can use logistics chests connected to the ports of the building to transfer bots into them once built. But you have the issue still of getting bots inside of a roboport within the building (without one already being there)---so i guess i see the issue. And if there is no player entity, don't know if factorissimo can work at all. (never tried).

Perhaps add a technology to request building supplies and construction robots from the surrounding surface? Logically build a construction-only roboport into the roof, with a buffer chest outside to supply building materials.

You can 'walk' into a factory just fine without a character.

I attempted to use it recently, My character icon spawned outside of the confines of the internals of the building and I had to revert to a previous save to escape. I'm putting this here to warn anyone else about this.

The control script probably restricts you to the explored area (on the default surface), even if you're now on a different surface.

That is correct. You can only move around so you don't reveal any unexplored area (unless you're in a vehicle). Edges of the map probably count as unexplored, so Factorissimo won't play well with BNW.
If you manage to drive a car into a building you might be able to move around, but it'll still be difficult to do anything inside a building. Well, unless you have a vehicle grid mod and bots/gear in the vehicle you drove into the building...