Hi greenrook,
I agree, the glass house whitefly is a stuburn customer to get rid of especially once they become well established.
I think the white bits on the underside of the leaves, may well be eggs or developing lavae, at one of their stages of their life cycle.
They work in a similar way to aphids and suck sap from the plants and cause the plant to weaken, the also exrete 'honeydew' which allows the growth of sooty mould to grow.
The eggs are difficult to kill with most pesticides but early treatment with 'Primiphos' or 'Pirimiphos-methyl' insecticdal soaps, 'Pyrethrum' or 'Permethrin' should prevent too much damage if applied early.
Spray heavy infestations at 3-4 day intervals.
Good luck and may you're avocado prosper! Tbird+