Unmapped may be best for the purists, as it discourages users from creating documents with a load of PUA characters, which do not display at all or which display differently if the font is changed.
I have long mapped Petite Capitals, Alternative Fractions, etc., to PUA code-points, because in earlier versions of FontCreator that was the only way to generate them automatically using Complete Composites. The latest version 10.1 now supports Complete Composites based on glyph names, so it is no longer essential to map them.
My DTP application, PagePlus, has an option to insert glyphs based on Glyph Index, which allows access to unmapped glyphs, e.g. Linotype Palatino Small Capitals, without using OpenType features. However, changing the font will change the glyphs that are displayed, which is not good. If I use the OpenType Small Capitals, there's not such a big problem. At worst, abc will be displayed instead of ABC small capitals.
I would advise going with unmapped. Most users could not care less about OpenType features, and don't know what the PUA is.
I see no benefit from imposing any constraints on the letter's contour width or the advance width. Only figures 0-9 and maths symbols like + = × ÷ have good reasons to be constrained to a single advance width.
There's a separate tag that can be used for Unicase (unic).
I think that users can use features however they wish. The font designer's job is just to decide on how to design the glyphs (with some particular use in mind perhaps), but not to prescribe how they should be used.