I find Taginfo statistics the most useful way to see “how X is really tagged” (more so than the OSM wiki). The 2 caveats are that taginfo doesn’t tell you what the mapper meant (it might not be the same as what you are mapping) and it can get overwhelmed by imports, or by one mapper’s insistence on using a tag that literally no-one else believes makes any sense (see https://wiki.openstreetmap.org/wiki/Tag:service%3Ddriveway2 for a recent example of that).
Guess why that isn’t a good idea. While there might be a place for a completely basic tag editor (but then you could just telnet to the API as ersatz), its usefulness for the general public just doesn’t really exist.
at least for basic typos like name spelt wrong, it would be really nice if you could just change the value on OpenStreetMap.org in the detail view without having to fire up iD (which is deactivated/hidden for small screens anyway).
if it is limited to changing freetext values like name, addr:street, operator, … it would be easy to implement and not require presets or other checks. Sometimes it is just a capitalization issue. We would be in the situation of having to weigh ease of mapping vs. attracting occasional vandalism by making it too easy. Surely if you could just tap on a business name in the map and change it directly there (or in the sidebar), it would make a perfect showcase how wiki mapping works. And a button “need to do more?” → Open this area in one of the dedicated OpenStreetMap editors like iD, remote control enabled editors, other editors
I’ve found Level0 is the fastest tool for adding tags and making corrections. You can easily start it from ie umap pop-ups; besides it is of common use for overpass-turbo users (OP has a direct link to Level0 in export section).
Anyway the missing validation is a little out of topic. I just need to understand if plural follows some british rules I don’t know or the shower example is just the result of a careless edits at the beginning of shower use in OSM.