Sticking with a single grading system is actually a conscious choice because it makes it straightforward to compare the difficulty of trad climbs. For example, if we added in the US trad system it would be hard to say how a 14 R compares to an E10 whereas if everything is on one scale it becomes much easier to compare. This is pretty essential for functionality on the site like generating lists like The Hardest Trad Routes in the World.
Another advantage of using UK grades is that they try to wrap all the difficulty (danger + physicality) into a single number, whereas in the US system this is split out. This then makes it kind of tricky because then you get difficult questions like "How does this 13a X compare to this other 13c R?"
The downside of this is that it does mean that contributors have to try and give an E-grade to routes that may well not have ever had an E grade before. Having said that we have found that the numbers from E-Grader are usually pretty accurate i.e. within the usual margin of error for grading routes.
We like to not be too strict in how we deal with nationality, so we use slightly loose criteria for when someone is included in a list.
In more detail, all climbers have a country where they are from which is used as the primary criteria for whether they appear on a list. There is then an optional Country of Residence field which can be used for climbers who live in a country long term but who might not have citizenship for whatever reason.
We do it like this because there are often climbers who have lived somewhere for a long time and are part of the local community, and excluding them on the basis of nationality feels weird.
Crags cannot currently be manually added, this is a known limitation and something we want to work on.
Crags are the source of location information so this can be quite a frustrating limitation, because it means there currently isn't enough information to answer geographic questions like "What are the hardest sport routes in America?"
This is by design and partly a matter of taste. My opinion is that slash grades suggest a level of granularity and precision in grades which does not exist.
While a climb may not have a slash grade we do record when a climber offers a slash grade as an opinion on a climb. For example a climber may suggest something felt "7c+/8a" which we would interpret as "it felt somewhere around hard 7c+ or soft 8a".
Having grade as a required property of a climb simplifies many aspects of the site. While it is not a completely accurate reflection of reality for some climbs the cases where it comes up are rare enough that this slight inaccuracy is a worthwhile trade off.
Where there are no grade suggestions available for a climb we estimate a grade based on available information like a climber's past performance, how many sessions it took them and anything they may have written about the process. We then indicate the grade is approximate and typically also add a note to the climb to re-iterate that the grade is approximate.
Also, I think grades are a pretty useful thing in general so this is a bit of my personal bias coming through.