Under eye hallows / loose skin under neck. How can I fix this?

Often the under eye hollows can be improved with surgery and/or fat grafting. Loose skin on your neck usually requires a lower facelift to improve. Best to see an expert for an in person consultation.

Thank you for your question. In general, reversal of the buccal fat reduction is carried out using fat transfer.
Because you were significantly surprised by the effect of the treatment you already had, there is potential value to a temporary reversal treatment in in the first instance, incase you dislike the outcome.
This could be carried out with fillers or with Sculptra.

In terms of the characteristics of the neck, it sounds like a neck lift intervention would be the appropriate intervention.

Unfortunately, no photos were provided. So, I can only answer in general terms.
HA fillers have been used for many years to treat tear troughs and under-eye hollows. For some people a robust HA volumizing filler injected properly and the appropriate location on the upper cheeks can help to significantly, secondarily improve the under-eye area without the need for direct injection in that location. For others, the direct injection in the appropriate locations of a low viscosity, malleable, cohesive HA filler is the required approach.
It is very difficult without photos to comment on the neck, except to say that I have been using HA fillers of various kinds to address all kinds of neck issues (alone or in combination with Botox), from laxity, to horizontal "necklace" lines and vertical neck cords.
Thees kinds of treatments should be left to the hands of experienced injectors and not novice, cut-rate venues selling treatments by the syringe and accustomed only to treating simple smile lines with ordinary Restylane or plain Juvederm.
You would be wise to consult with an experienced, board certified cosmetic dermatologist filler expert. Hope this helps.