Is it possible to change the “Done” icon button to arbitrary text?

For iOS using PESDK.bundleImageBlock is the simplest way to change any icon, the only way to get the toolbar to the top is to embed the editor in a UINavigationController (see also our example project and PhotoEditViewControllerOptions.navigationControllerMode for more advanced uses), and for customizing the apply button you would use the PhotoEditViewController.applyButtonConfigurationClosure of your Configuration object, e.g.:

let configuration = Configuration { builder in
builder.configurePhotoEditViewController { options in
options.applyButtonConfigurationClosure = { button in
button.setImage(nil, for: .normal)
button.setTitle("Done", for: .normal)