If Former Champion’s STR is 1 or lower, it will gain both icons and keywords.
Setting Former’s Champion’s STR to a specific number with cards like At the Palace of Sorrows or Strangler will give it the corresponding challenge icons and keywords, regardless of how much power it has. In the case of Strangler, it will be too late to use stealth if Former Champion’s STR was higher than 3 before it was declared as an attacker.
It is possible for a character to gain a keyword while another keyword is being processed. For instance, if Former Champion’s STR is lowered after gaining power for renown, it is possible for it to gain intimidate during that same challenge. You can then use that intimidate if you have not already used intimidate in that challenge.