I believe users cannot close/delete their posts after 2 hours. It will need closed by a GLPI support agent (not me).

Do you have an image that shows this counter? I don't recall that being in 9.5. Is it possible you had a plugin that added it?

This is already possible by adding "Assigned to a supplier" as a hidden field in the ticket template (Templates button at top of ticket pages).

Done for GLPI 10.1.0.

Related PR:

https://github.com/glpi-project/glpi/pull/14403

You can use tasks with the state set to TODO for this. That also means that ticket/change/problem templates can have multiple TODO tasks added. You can click the checkbox on the TODO tasks to mark them as DONE.

Tickets can be imported using the GLPI API.

ChromeOS supports Android apps and there is an Android agent. I've not personally tested if it works though.

This was added in GLPI 10. You can hold the ctrl key down when clicking on the column headers to add additional sort levels.

The referenced value is the system default. Each user has their own preferences that can override that setting.
Can you verify that your user has the setting set?

https://glpi-user-documentation.readthedocs.io/fr/latest/first-steps/preferences.html#personalization-tab

It is currently possible to do this indirectly using SIM cards.
For direct linking, see:

https://github.com/glpi-project/glpi/pull/11299