PPD

Question
I had 2 teils, brother & Sister who would have been 3 yo this August.  My female suddenly lost feathers on both sides of her neck, poop was hanging from her vent and fluffing up.  Took her to a so-so vet that does a little with birds-found she had a heart murmur.  At that point she was being herself, but became so weak when she tried to fly she'd crash and be out of breath.  Took her to another vet who was amazing but couldn't pin point what was really happening.  She died in my arms in March.  Two weeks later her brother starting showing signs of what I thought was depression.  (I waited another week and did the dumbest thing-bought another bird when I don't have room to separate them.) The brother of the one that died started showing the same signs as his sister, loss of feathers around the neck, poop from vent, fluffed up, went off his food and started rapidly losing weight.  And off to the vet we go. Blood tests revealed all these little things that pointed to bournavirus and he was put on celebrex, there has been no improvement. I've also been giving him benebact which helps him lot-he will then eat some rice and nibble on seed. I was force feeding him baby bird food mixed with crushed pellet and jarred baby food. He will longer eat these.

The new bird is 6 months old and showing signs of lack of coordination, having trouble with his landings (not clipped)and just little tiny things that seem off.  I swear I saw him have partial seizure-standing on his rope, 1 foot went out to the side and his toes stared to open and close, and when I went to talked to him, there was no response.  This is about the third time this has happend. Now he has started to fluff up but his poop looks normal.
I'm off to the vet again and more dollars - having blood work done This Thursday on new bird and putting the other to sleep.  He is not getting better @ starts trembling when I go near him and I just can't do this to him any longer.
The brother and sister ate mainly seed, with a few vegetables but not much.  They attacked their cuttlebone when babies but seem to stop about 1/2.  Is this normal?  I know they need a lot of calcium.  I also tried different mineral blocks but none of them will touch them.  The new guy eats vegetables, nuts and some fruit but will not touch cuttlebone and mineral block (I tried to show him how good they are).

Since this virus is now in my apartment how do I get rid of it?  How do you disinfect carpet without toxins?
Do birds stop eating cuttlebone as they get older?
can birds have seizures?
Do you think it would help if I started giving new bird benebact to keep his digestive tract moving?
What blood tests should I have done?

Sorry this is so long but I think I had to get this out.  I'm devasted on the loss of my girl and putting her bro to sleep.  I'm going to try everything to keep the new guy healthy.

I'm pretty sure PPD is killing my birds.

Thank you for your time

Answer
--  One of the deceased birds should have absolutely had a necropsy done and that would have been able to tell you what's happening - or at least given a 'good idea'.

--- I am very sorry for your loss.  It would devastate me.  ---

PDD is gone the minute the infected birds are gone.  This virus is not very stable in the environment so spreading is believed to require close and direct contact.   Although it apparently also includes being in the vicinity when an infected bird sneezes or shares food.

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The one thing I strongly recommend against is trying to avoid an illness by treating for it before it's diagnosed.  I'd rather you just feed a super healthy diet and

 Check here for more on feeding for a stronger, healthier bird

  www.4animalcare.org/birds

Again, I'm heartbroken for your losses and can't imagine the pain you're in