My project advisor responded back to me stating that their engineer is confident that 50A can be added into my backup panel, and nothing into the main service panel. All of the electricians I contacted so far are refusing to add anything into the backup panel, stating my load calculation is already at 89A and adding anything will overload the panel.
Is there a list of Tesla recommended electricians who may have more knowledge of powerwall backed up system?
I don't think the 50A in the backup side is overloading the panel rating, it's just overloading the 100A breakers that Tesla somehow put in over the top.
Maybe you can search your local solar companies that at one time tried to get Powerwalls only for Tesla to tell them to sod off with zero inventory. Those installers would have been trained to do the Powerwall, so they'll be able to get your HPWC in since the Powerwalls won't be so scary for them.
I had Sunrun as my PV+ESS installer and also had difficulty getting a HPWC in. I had to find a smaller solar shop to add in my HPWC on the backup side. The guy spent all the money to get ready for selling Powerwalls and did all the Training, but then Tesla cancelled the deal. So he seemed much more comfortable adding the HPWC than the normal electricians. But I didn't have the problem you have of a throttled backup loads panel. Sunrun put in many 200A breakers all over my system so my backup loads panel could get 200A easy.