Hello!
- The HSM SIM card will be locked, and the only operation left possible to revert this state to my knowledge is to run its firmware update. See this topic for details: Initialize Nitrokey HSM Device Blocked - #10 by sc-hsm.
- Yes - both PIN counters and the actual secrets are taken from the smart card. Technically there should be no limitations in this regard (e.g. like there is no SIM card <-> device lock or authentication required). Just a fair warning: hardware changes makes warranty void, can result in hardware damage in some conditions (e.g. ESD), and we do not support it. Please keep in mind some components might be not compatible, or some features not supported by the replaced SIM card.
- Without the SIM card Nitrokey HSM2 (and Nitrokey Pro as well) will not start, and with this it will not show up as a USB device. This is by design. During the boot the serial number of the device is received from the smart card, and without it device will simply abort the sequence.
- In general yes. Some internal smart card communication procedures (e.g. getting the serial number during the boot) would have to be removed to continue to the USB enumeration, otherwise it should be possible to read out compatible SIM cards (using supported ISO T=1 communication protocol AFAIR).
Nitrokey HSM and Nitrokey Pro have a lot in common, and in fact use the same repository of the latter: