Just did more testing today, 12 secrets is the absolute minimum # required for S+; 11 secrets and 0 heals results in S rank. Thankfully, 12 secrets and 1 heal still counts as S+, so we have at least 1 safety heal for our runs; we can cut out BOTH the acid bottle and the shotgun entirely from the run at this point.

Just tested to see what the minimum number of secrets for S+ rank is, 13 secrets with 1 heal also counts for S+, so the S+ Rank Restricted run can cut out the Acid Bottle entirely. Pretty sure each secret approximately equals 2 or 3 heals, so 3 (4?) heals max at 14 secrets, 6 heals max at 15 secrets.