I don't know if onSaveInstanceState() is really possible to implement well without serializable continuations, but other Android REPLs work pretty well (particularly Ruboto IRB!), so you might find some ideas there.
"autocomplete keeps trying to remove the space before commas"
Autocomplete ruins my day too. ^_^ You might be able to go to "Settings > Language & keyboard" to configure it. If it still happens after that, installing a new keyboard oughta help. At least it'll give you a fresh set of quirks to deal with. :-p