I have conducted some research, and here is what I can come up with:
Thanks to Phase 1, since humans die shortly after mating, this will greatly reduce their average life expectency. Which in turn would greatly hurt human productivity since shortly after reaching adulthood they die.
The virus has developed resistant to the strong acidic content of the hosts' stomachs
There are bacteria that do pretty much as you described in real life so this is completely reasonable.
The animal's droppings house colonies of the retrovirus which become food for other 'metahumans'. Believe it, this is survival instinct at its finest!
The plants thank you for your donation of nutrious droppings. By the time the baby is born (assuming 9 months), the droppings likely already have been absorbed by the dirt, washed away by the rain or other post apocalyptic weather, or eaten by insects (ever seen an apple that has been outside on the ground for a week?). Your baby likely will never be able to get them. Also if the droppings contain concetrated nutrition then it will be robbing the fetus of nutrition that it needs to grow.
Lots of DNA
I am assuming that the retrovirus in Phase 4 is produced during Phase 1 and it contains the genetic material of both parents so that it can create sperm and eggs in Phase 6.
Note that the retrovirus can either have a full copy of the parents' genome or only half of each. If it has a full set then the sperm and the eggs can have a greater variety of combinations, which is extremely valuable since the parents will only get to mate once, and you want as much genetic variety passed on to the next generation.
As such a single human has approximately 3 billion base pairs. Since your virus contains two humans worth of dna it will need to be able to store 6 billion base pairs. The largest virus I could find has 250 thousand base pairs. This means you are going to be trying to store 24,000x times as much material into a virus than typical.
Size matters
A large virus can reach 250 nm in size, while the nucleous of a cell is 3,000-10,000 nm (and a human red blood cell is 9,000 nm). Odds are your virus will be closer in size to a nucleous of a cell in order to store the shear volume of DNA it is trying to carry. Viruses typically infect a cell and have the cell produce lots of copies of itself till the cell bursts. With a virus so large an infected cell likely will only be able to make a single copy of the virus, or two eggs and two sperms before it bursts. However, a single copy is not good enough. Virus only can infect a cell once, since it injects its only copy of its DNA into the host cell. This combined with a single copy results in it not ever being able to make more of itself.
Phase 7
Lets put the cell size issues aside and look at the animal size issue. You got lucky the creature you infected was a carrion eater and has successfully produced a ton of eggs and sperms. Human fetuses have a 9 month gestation period, and need a host that can carry a fetus to term before it dies. Also you will need to either infect multiple carrion eaters in one go, or the one carrion eater needs to be capable of delivering multiple metahuman babies before dieing. That rules out a number of carrion eaters, since they are birds and the weight from the fetuses would eventually prevent or hinder their ability to fly and thus they will die from starvation as the fetus(es) sucks the life out of them. There are so many small (insects) to medium sized (coyotes) carrion eaters out there that odds are they will get to the corpses first before a large enough carrion eater makes it to the scene. If this happens then your metahumans will fail to spread their species further. Since even a coyote would likely not be big enough to carry a fetus to term.
Phase 8
Lets assume that the carrion eaters in your post apocalyptic world are much larger and can handle the multiple fetuses long enough for the babies to be born. Well congrats you have triplets! Unfortunately you now have babies without any parents. Humans need a lot of help growing up. They lack defenses and can easily be picked off by a post apocalyptic predator. Human babies like other mammals rely on milk from their mothers to feed them, but unfortunately mom died during conception so no milk.
Ideas
With all this said, you might want to upgrade your thing from a virus into a flesh eating bacteria that can cocoon its victims. That may deliver what you are looking for. The flesh eating bacteria converts the host essentially into an incubation chamber for the fetuses and near the end it turns what is left over into a nice juicy paste that the baby can easily consume after being born to give it a quick jump start rather than relying on nutricious dropping.
Second idea is to skip the virus and have fertilized spores being produced during phase 1, which act like seasoning on the metahuman corpses. The spores get into the intestines and leach off the nutrition that the host eats until they get too large. At this point the host is likely rather weak and the embryos then devour their host from the inside out, where hopefully they get enough material during that last phase to be properly born.