Learn English – Is this usage of “now” correct

Yes, those are correct usages. Use viz. just as you would use namely. Wikipedia's examples:

• The main point of his speech, viz. that our attitude was in fact harmful, was not understood.
• "My grandfather had four sons that grew up, viz.: Thomas, John, Benjamin and Josiah."
• The noble gases, viz., helium, neon, argon, xenon, krypton, and radon, show a non-expected behaviour when exposed to this new element.

Viz. is short for the Latin videlicet, which means namely. Though I am a native English speaker, and I would not find this weird or inappropriate, it is uncommon, and most people would have to look it up in a dictionary. It would be better to use namely or that is to say instead.

Edit: I find this Ngram rather informative, too. Once, apparently, viz. was more popular than namely:

Most of the modern usages of viz. are not the abbreviation in question, but nearly all the older ones are.

I think, in this construction, if is essentially equivalent to although. One can substitute although for if, salva veritate, in the examples given, and in all the examples I can think of.

I might add that this particular if construction is an "aside", often set off by dashes or parentheses in writing, or otherwise almost always by commas. All of these punctuation marks indicate that the tone and volume are low, with flat intonation through the phrase, which is a mark of a part of an utterance that's not meant to be parsed as the main predication.

"Aside" is a stage direction for lines to be delivered not to the other players but to the audience.