Any second language used only for programming purposes is going to be doomed from the outset anyway.

Not if it's consistently used by everyone.

I work in a Chinese engineering firm. They work with Chinese people (and me). They sell their products to Chinese firms. What possible incentive could they have to make all their engineers use a different language than Mandarin to communicate in?

They don't and that's fine.

If they grow to the point that international markets are a concern, they'll have to i18n their products anyway and given the costs of that, updating the design documents in another language is a minor cost.

Yep, including conlangs.

Conlang IALs are a solution in search of a problem for an overwhelming number of professionals.

In search of what problem?

They present a high-cost initial barrier of entry (the time it takes to learn the conlang to fluency) with a very low payout in the short- and medium-term for almost all involved people.

Where's your proof?

Learning English and Mandarin also has a high-cost initial barrier of entry; IALs are however better designed for inter linguistic speakers.

And even if the engineers in question did learn the conlang do you genuinely believe they'll use it when doing work among other speakers of their own language?

yes.

Do you genuinely believe the conlang will be the primary communication tool?

IALs, yes.