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The missing 4.2(x) and 5.1(x) images are likely in restricted/legacy access. Please check the Cisco ACI upgrade path tool or raise a TAC case to confirm the correct supported path and obtain the required images.
With RSTP, root bridge changes usually converge in under a second to a few seconds on a clean link. It only takes longer if legacy STP or more complex topologies are involved.
You’ve set it up pretty well so far. For fullscreen and disabling touch, a kiosk browser app might work better than Chrome since those usually have more lock-down options.
