实在 and 确实 mean more different when used as adjectives but not too much in adverb usage as “indeed”. However, knowing the meanings of their adjective usages would help on differentiate their tiny differences in adverb usages
Look at these examples in adjectives:
- 他做事很实在。 He‘s very honest when he works.
- 他做事很确实。 He’s very detail-oriented when he works.
Therefore, in your sentences, if you say 她实在很漂亮, you’re saying you honestly think this is true.
If it’s 她确实很漂亮, you’re saying objectively that she is a pretty person.
行不行 is more of a fact than an opinion so 实在不行 and 确实不行 are the same.
Hope this is helpful for you!