please advice an EE idiot on using chargers

[Jim Horn]

I would recommend a combination of your choices #1 (external charger) and #2 (modified battery holder). The modified holder works well with today's much higher capacity NiMH cells so you can easily have one set charging while another is in use - and the charge will power the calculator far longer than the original battery packs would do. Double win!

Why I don't recommend your choice #3 is that lab bench DC supplies are more expensive, larger, can easily have their voltage accidentally turned too high, and connecting them to the calculator's battery contacts can damage them, risks reverse polarity, precludes portability, and is inconvenient.

Best to you!