Help! My cat hates her new "friend"

Hi fellow cat fans.  About 5 years ago, my husband and I adopted 2 female cats that were 10 months old and litter mates from a local rescue group.  They got along well and were so cute with each other, cuddling and grooming and all cute things siblings do.  Unfortunately, about a year and half ago, one of them fell ill and had cancer so we had to put her to sleep.  My husband and I travel a lot for work and so we felt bad leaving our remaining cat, Megan, home alone so much so we said that when we moved into our own home, we would get her a friend to keep her company and play with her so she would lose weight (she gained 4 pounds since the passing of her sister). 

Recently, we moved to a new city and found a rescue group.  We went to a foster home to meet 2 of the possible candidates but my husband wanted to take another cat home who was being bullied by one of the other cats in the foster home. 

About 2 weeks ago, we brought Buddy home for a sleepover.  He's a year old neutered male and he's very sweet and use to being around other cats as he lived with several cats in his foster home.  I put him in a safe room for about 3 days and my cat would hiss everytime she passed by his door.  The evening of the 3rd day, my husband thought it would be a good idea to put Buddy in his carrier and show him to Megan.  Megan just hissed and growled at him.  The next day, my husband wanted them to at least be in the same room together to start getting use to one another and Megan hissed whenever she saw him and felt he was too close-even if he was 20 feet away.  I kinda felt like we were making some progress but we had to go back to work so we returned Buddy to his foster home while we were out of town.

About a week ago, I picked him up again and put him in a safe room.  After a couple hours, my husband wanted to let him out of the room so he could begin to acclimate himself to our home again.  Ever since then, I feel like there's a Nat Geo episode constantly going on in my home.  Megan is the sweetest cat until she sees Buddy and then the hissing starts.  As soon as she doesn't see him, she walks around the house looking for him and she's totally fine until she spots where he's been hiding and then she stares and hisses a little.  We've had a couple skirmishes where they've "surprised" each other and Megan hisses and chases Buddy away from her.  I don't think they've had any contact as Buddy is much faster and more agile than she is.  We try to distract her with toys and play time but when she's in her mood when she sees Buddy, nothing else phases her.  We've been trying to scent swap by using a sock and she just sniffs a little and walks off.  I've tried scent swapping by brushing him and then letting her smell the brush and she just hisses.   I've even tried the vanilla trick where I've put vanilla under their chins, between their shoulder blades and at the base of their tales but to no avail. 

Currently we have buddy set up in our office with glass panels in the doors so he can see out and Megan can see in.  They are ok in the same room as long as they don't get too close to each other or else Megan freaks out and hisses and chases him off and he hides in his safe room for a while.  Should I put him in a room where he can't see out and Megan can't see in?  What else can I do?  Is this relationship doomed?  We really don't want to take him back to the foster home because there was another cat that was being really mean and nasty to him and scratched him.  I just don't want a household with two forever unhappy cats. 

Sorry this is so long-winded.  I'd appreciate any input from anybody who might have some good advice as we really want this to work out or at least for them to be able to live together when we're gone without feat that they may kill each other.  The best case scenario is they become friends and entertain each other while we are gone and they are home alone. Megan is really testing my patience with her behavior. 

Cats do not like change.  The older they are, the more they hate it.  It's a common thinking pattern, that cats are just like dogs and want a "friend" to keep them company.  The truth is that cats are not "pack" animals.  They can be social but they don't feel the need to belong to a pack or have a friend like a dog does. Because they don't like change, it can take them several weeks to accept newcomers.  So hissing, growling, etc is to be expected.  Keep interactions brief and do not "punish" for bad interactions, just calmly separate them and try again later.  Treats and food can be very helpful.  Offer treats for each good interaction.  That way, she starts to associate the new kitty with something positive.  It can speed up the process.  But, don't push it and don't get discouraged just yet.  It can take a month or even a couple of months to see some noticeable difference and acceptance.