Should the inverse statements also be editable in the other item? Generally I think this idea can be useful, although some items will become really huge. We should specify a list of properties that should be shown and others that shouldn't be visible. Adding hundreds of statements to Q2 for example will make that page inaccessible.

This needs considerable discussion if and how to do it before someone starts working on it.

Maybe just load the first ~20 statements with a "Load More" button?

Incidentally, this would less than double Q2 (71 statements, linked to by 68 items)

How about adding a link to query server that lists them?

It's something that could easily be done by editing the interface directly at www.wikidata.org

There are several ways how to deal this need. They can be developed paralelly or even synergistically:

• to modify the current "Special:WhatLinksHere" function - to sort and group links by property which uses the item as a statement value (the easiest way to start)
• to develop quasicategories - reciprocal properties (properties which use and link other items as values in their statements) or even all properties can be managed in similar ways as classic "categories" do - i.e. such links can work similarly as "category tags" do (and similarly as hashtags work usually): the list of items which link to the same target item using the same property can look and work similarly as a category page. Where statements forms a logical hierarchy, some functions similar to Hot-cat and Cat-a-lot can be implemented to facilitate the maintenance of statements. This may be a next phase following the first step (can be gradually developed from the "Special:WhatLinksHere" function).
• to create virtual statements - values (properties) which could be called in the same way as classical directly filled statements, but in reality it would be a calculated function. It can apply for automatical mirroring of reciprocal/inverse statements, but also for some other cases, e.g. to calculate "region", "municipality" or "province" from the chain of P131 statements, or certain level of classification from the biological taxonomy hierarchy. There are two options to consider: the values calculated and displayed automatically before they are called, or the values calculated only when they are called. If Wikidata can do this directly, it will not be necessary to program it separately for each sister project and each infobox.