Question: In particular, I don't mean mere alternate spellings like *colour*, *honour*, but words that are entirely different: using *lift* instead of *elevator*, *fridge* instead of *refrigerator* etc.

What is the common outlook on using them in plain informal speech in the US? What are the chances they won't be recognized at all? Will they be seen as pretentious? Weird? Unwelcome?

Answer: 
I think there is a chance that some of the less common words would not be recognised at all between the AmE and BritE, but it does depend on what sphere the words are from.

The common ones are fairly well known, others not so:

From cars:

Trunk, boot. Hood, bonnet. Rocker panel, sill etc

From culinary:

Bitter sweet chocolate, plain chocolate. Tomato sauce, crushed tomatoes. Tomato paste, tomato puree. Cilantro, coriander.

And so on.
In general, I would say no, but there are some exceptions. The first time I encountered a sign in a public place saying "toilet," I felt it was quite crude. Americans would only use that word for the actual fixture, preferring restroom for such contexts.