I would set them up with decent inbound liquidity at the beginning and explain only later if it ever becomes an issue. Remember, they will likely not have that many customers at the start who pay with this. About the question of exchanging to fiat, I would again help only once that becomes a need for them. My general strategy is to hook them up at the start with a basic solution, and solve obstacles later.

I have guided small number people (not merchants) in the past with either onchain or lightning, and I never regreted going the self custodial way (what I see as the correct way).

What I found surprisingly easy was explaining backup seeds. Since it is a physical thing (piece of paper), people alredy have an intuition for how to work with it. Explaining this part well is imo important. Put emphasis on "Do not loose it or you will loose your money! It is a secret!"

Phoenix does backup seed and also it has the splicing function, which is their way to enable for onchain and lightning with a single app. Imo the best for noobs. Disadvantage is high fees.