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This looks great. If we use the 'Default Variant', will Intl.DateTimeFormat and Intl.NumberFormat work properly for formatting dates and numbers in US English?

Intl will not be available in default variant (similarly as with the old JSc used by React Native). You could use Date.toLocaleString or Number.toLocaleString though, which have "fallback" implementation that does not necessarily match "en-US" implementation from ICU (specifically for dates)

e.g:
with intl Date.toLocaleString will return the following format for en-US: 7/12/2017, 4:27:45 PM
w/o intl Date.toLocaleString will return sth like Wed Jul 12 16:24:54 2017
when you call Date.toString you will get Wed Jul 12 2017 16:24:54 GMT+0200 (CEST)

We recommend that for number/date formatting you use an external library if you opt out from using intl variant.
The biggest difference for the intl variant is that it supports collation (that is proper comparison of non-english letters). By default strings are sorted using UTF-8 codes of subsequent characters, which is not the way it works with languages that have non-english letters.

Good to know, thanks. I'll take a look at using the Intl polyfill (https://github.com/andyearnshaw/Intl.js/); I'd like to use the standard APIs since they'll work well on iOS too.