Thanks for the flowers Fraser, but there are other hams who have done more scientific research in this area than I have (e.g. HB9BCB).
Just ignore unsupported statements. If you really want to know what’s going on, make a model with the setup you’re most comfortable with and the bands you’re most interested in. Then do some variations and see what changes.
Yes, compare the radiation pattern of a 2λ with a 1λ in an inverted-L (or more precisely inverted-7) configuration for the 10 m band in my 7-band EFHW document on page 30.
Yes, check the above document on page 30 for a 1λ EFHW on 20m with the same configuration. I can confirm that the model matches reality.
73 Stephan