Another key thing to consider in your Rondon/Lopez comp is that Lopez pitched in the DSL, and only start pitching stateside at age 19. Rondon was a top college pitcher. The Nats are fine to be cautious and slow with him. I myself, doubt he sees BP time, but your points aren't totally off-base. The FO likes to run arms as far as they can as a SP before making a move to RP. Purke was still used as a SP here, even after being DFA'd. Solis only started pitching in relief last season. Trienen was still getting SP looks until he was 27.
I'm not trying to compare him to Rodon. For.one, I'm talking about Lopez relieving, and Rodon is starting. I merely brought Rodon as a quick reference to the fact that pitchers absolutely pitch at the big league level with 3rd pitch and command issues.
Also, the in house guys you mention were made permanent relievers. That's not the case here, so I don't really see the relevance. We're talking about a short term thing to possibly help the big league club. Think Finnegan last year with the Royals. I know the narrow mindedness of slate will have a one liner about Finnegan being a lefty, but again, that's not the issue. A young fireball er can come in and have success and give the team a shot in the arm late in the season is the goal. And it doesn't mean it has to stunt their long term develooment.
Also, I dont know if you guys realize, we're probably talking about Lopez losing 2 or 3 starts. If he's getting shut down around 125-130 innings, I'm saying after about 110, move him to the pen. Let the next 20-30 innings be in relief. I just don't see how "missing" 2-3 starts, while still pitching, stunts ones long term growth. Any potential loss, is easily matched by pitching and having success at the big league level. The confidence that comes with that is a big plus. Now obviously that is an assumption of success, but I assume that because if he's not have success, then you don't use him.
I say you stop him around 110 as a starter. Have him throw a pen on his usual schedule, but instead of a start on day 5, have him come out of the pen(still in the minors btw). Instead of his usual in between pen, have him throw in relief again. On day 5, relief again instead of a start. That gives him a small taste of what it takes to be a reliever and you can see if he has the mental abikity to switch gears at this stage. If he doesnt, he's still on a routine where you can still start him again in the minors for a couple more games. It would be very similar to skipping a start or two mid season to stretch him out, like the Pirates did with Taillon a couple weeks ago. Or you can shut him down and just continue to work on the side. I wouldnt just toss him into the bigs without a plan.