1. Redster CS has a lower instep height, narrower heel, narrower ankle but if you fit an Ultra without issue it will be minor work to turn the CS into an all-day comfortable boot.
2. Cuff is slightly lower on the Redster CS, but we're talking single-digit mm. Liner tongue height up the shin is the same and that's basically what you feel. I haven't missed my Ultra in this regard in the slightest.
3. You should be fine then. If anything, it is a much more supportive boot and as such will place less strain on your Achilles (in other words, it does more of the work for you).
4. If you want a heavier/burlier pair of Hawx Ultras, you end up getting the Redster CS