Two questions. Kindly answer if Ra would be tafkheem or tarqeeq when:
1. Ra with shaddah has fat-ha but preceding letter has kasra or vice-versa (e.g. min rabbihi, la nufarriqu)
2. Ra with sakin is preceded by another tafkheem letter with kasra (e.g. in uHsirtum)
Wa alaikum assalaam wa rahmatullahi wa barakatuh
When determining the rule for the ra' which has a shaddah, we do not look at he saakin part of the shaddah, instead we only look at the vowel on the second part of the shaddah, both the saakin and voweled part of the shaddah have the same rule and they are not split up in the rule.
So in your first example: min rabbihi ; the noon merges int he the ra' and there is a shaddah on the ra' with a fathah, there is tafkheem of both parts of the shaddah
In the second example: la nufarriqu, there is a kasrah on the shaddah of the ra', so we read both parts of the shaddah with tarqeeq.
2. You can find the answer to your last question in the rules for the ra':