I did not know that! I was really getting hung up on the "not having any non-seniors" point.

I second this. I lead a team of engineers, and to us the main dividing line between senior and not senior is if you're able to take on a project and lead it autonomously. I.e., you've gone past the stage where all you do is take on the next ticket in your task tracker; you have an awareness and understanding of the bigger picture, which allows you to create tickets on your own and select the most appropriate thing to work on next. The lead (me) is still there to help prioritize, fetch requirements, unblock things, etc, but it's fairly light touch management.

(Edit: my job title is Principal Software Engineer)

Definitely. My thinking is that it would make it harder for HR to undervalue the position and, if we ever do get to hire additional developers, they will be looking to me / whoever is already in my position, in a senior / junior type fashion - at least for a solid chunk of time.

I can definitely see what you mean, and I appreciate the detailed post!

As for your scenario, I feel as though my heart rate would go up to a moderate amount, maybe less depending on the exact structure of the project.

The reason I say that is because I've done similar things in the past. My wife also has a job as a junior developer (working from home as part of a smaller multi-dev team supporting a massive project), but her team is set up in a way that she can only get a few minutes a week with her senior since he is always busy with his own tasks. She was never given documentation or even a walkthrough to explain how their project is structured or how their database is structured (they said in the interview that they would do this, but in reality the senior has to spend all of his time on other projects or in meetings). There are times that she gets stuck on a task that she has been assigned because she doesn't know where in the source code she needs to look and she can't get a hold of her senior. She'll ask me for advice (without looking at the code, it's essentially a 20-questions type scenario of hypotheticals) and, even though it is a completely different programming language and I haven't actually seen the code, I can figure out what it is that she is needing to do and help guide her to where she needs to go in her project. When she finally gets a chance to talk to her senior about it a few days later, he confirms that the steps she ended up taking were correct for their set up.

For example, she once had to update the items in a drop down that she couldn't find in the code in their project. She knew that the dropdown was being loaded on a specific page, but the items weren't being populated in that same place. I assumed that this was likely using a stored procedure or a view on their database to pull the dropdown items. With that assumption, I was able to help her trace to where the data was being loaded in their data repository. That gave her the name of a stored procedure. She went and updated the stored procedure's definition in their dev server and it corrected the dropdown. She later met with her senior and he confirmed that the dropdown (and many other features in the app like the dropdown) are pulling out of stored procedures so they don't have to redeploy code to update things like that.

Similarly, one of the projects I had to work on at my org is actually taking an MVC web application and an API web application that are written by one of their multi-dev teams that are provided by our ERP software provider that we can customize to meet our specific needs. I did have some documentation to go off of, but I was able to get that loaded in and making customizations that fit within their architectural style within a couple of hours when I first started it.

I appreciate your advice! I'll definitely work on getting the hard numbers to help my supervisor out. She's been pushing for this for a while, so hopefully I can provide some materials to aid her.

I've had to do this before for a former job where I went from a Team Lead on a Helpdesk that started coding tools to help improve our agents metrics and the availability and accuracy of reporting to management and agents. Our helpdesk had no developers, but I was able to show how our team went from spending ~10,000 man hours a year on manually running reports to send to agents and management and only having the reports available weekly to having all of it automated and updated daily for everybody that needed them. So, I can adapt the methodology I used for that to the current org as well.