Is Particle Mesh appropriate for offline?

I have a project I’m working on which I would like to use openthread or similar for.

This project will be 99.9% offline. It will only connect via wifi hotspot of a technician occasionally for firmware updates. I would also like to be able to update firmware from the “gateway” via usb and have it propagate through the network.

In this system there would be one main unit which will control the rest and act as a central “gateway”(not usually internet connected) and 4-60 distributed sensor nodes.

Can I use the OTA firmware upgrade without having a fully internet connected device via USB, BLE or hotspot?

Can I commission nodes onto the network without internet?

Are there any significant issues having a regularly offline mesh system?

Unfortunately this isn't going to work well with Particle mesh, which is intended to be used online, and only sort of works offline, with limitations.

Can I use the OTA firmware upgrade without having a fully internet connected device via USB, BLE or hotspot?

That's not possible. OTA firmware updates are only possible from the cloud. The mesh gateway is only an IPv6 router.

Individual devices can be updated over USB or BLE at the device. Devices elsewhere on the mesh network can only be updated via the cloud (Internet or hotspot).

Can I commission nodes onto the network without internet?

No, new devices can only be added to the mesh network when the gateway is connected to the Internet.

Are there any significant issues having a regularly offline mesh system?

This is not the intended use case, so you will likely encounter difficulties.