Center Controller reported the need to intervene after witnessing a Radar Controller have difficulty sequencing an approach resulting in an aircraft descending below the minimum IFR altitude.

Narrative

An R Side was working sector X. I was training a developmental on the D side. Sector Y calls to approve Aircraft X on the RNAV approach to RWY XX at ZZZ. This approach is a rare one that that requires user authorization to do. This is something that most controllers are unfamiliar with and generally uncomfortable with. We approved Aircraft X to be routed for the arrival. Sector Y gave the routing. We did not get communications as quickly as we might have wanted; but they did come over in decent time. The R Side working was clearly uncomfortable working multiple ZZZ arrivals. He offered a speed restriction to Aircraft X but the reduction was not enough. I told the R Side that he needed Aircraft X to slow more than he instructed. The R Side offered a new speed. As Aircraft X came up on the IAF for the arrival it was clear that the two aircraft were going to be close; but it still would work.By this time I had stopped training the D side (unofficially) so that I could train a CPC on how to run a sequence into ZZZ (unofficially). I instructed the R Side to continue to step Aircraft X altitude down. I instructed him to call the proceeding traffic to Aircraft X. The proceeding aircraft had gotten a visual and I thought Aircraft X might get the proceeding aircraft in sight if he could get low enough as the ceiling was 6000 ft overcast. The R Side descended Aircraft X to 13000 I believe; but as Aircraft X hit the IAF for the arrival he started to descend. I had to bring to the attention of the R Side that the aircraft has left the assigned altitude. The R Side recleared Aircraft X to 12000. I had to then instruct the R Side that the MIA (Minimum IFR Altitude) in the area was 12500. I had to then instruct the R Side to issue a low altitude alert. He said 'Aircraft X low altitude alert.' He did not know the rest of the phraseology. I had to instruct the R side on the remainder of the alert phraseology. I then had to talk the R Side through the remainder of the sequence as there were additional arriving aircraft at ZZZ. Aircraft X appeared to descend below the MIA until he was recleared. I don't even know where to begin. First; and absolutely foremost; ZZZ needs an approach control. I don't know what person in this agency refuses to hear this and accept it; but that sure is what continues to happen. For a long time this facility had good controllers who were very capable and safe. Clearly; from reading the narrative above; you can tell that this is no longer the case. The individuals inside this area are simply NOT of a satisfactory skill level to work this kind of traffic. There are still some here who can do it; but by and large; this area does not have the type of CPC's who can do this safely or the amount of said type CPC's. Secondly; this area has been working mandatory overtimes for nearly a decade. And this isn't ZZZ Center where we work OT's cause we demand the money but barely sit in front of a scope each day. We are routinely one of the busiest areas in the NAS; perpetually understaffed; and are getting absolutely obliterated here. One of the unintended consequences of this is that this facility; and the burned out controllers therein; will literally checkout a corpse. Again; see the aforementioned narrative and tell me I'm wrong. The training here has lost ALL meaning. It is a matter of filling hours and checking boxes. The notion that any individual needs meet any kind of standard here; let alone a high one; is absolutely laughable. Thirdly; this facilities management has instituted the 'more in than out' rule. The rule itself is ridiculous; but there ain't anything anyone can do; or is willing to do; about it. Much like the unintended consequences listed prior; controllers are made to sit on dead sectors devoid of any meaningful traffic to either train on or keep their skills proficient with. Just last week I sat on a sector for nearly twenty minutes that had zero aircraft in it. ZERO. Justification was 'more in thanout.' The R side for this session comes completely undone anytime the sectors are the tiniest bit busy. If this area had the ability to break sectors off and the staffing to accommodate that so everything so that everything could stay 'green' that would be one thing. However; that isn't the setup here. Sectors are 000 to moon stacked next to each other with nowhere to offload. The result is controllers sit on sectors that simply too busy because 'we've done all we can do.' This facility has become as dysfunctional as any could possibly be. Good luck with fixing anything in a facility who's management cannot and will not acknowledge the risk and toxicity that they are fostering inside the place.