Let's take it from the top.
- Yes. Metamorphosis, as per its description:
You assume the form of a creature of the same type as your normal form, or any other type except construct, elemental, outsider, and undead. The assumed form can have as many Hit Dice as your manifester level, to a maximum of 15.
If a metamorphosing creature is any of these four types, it can transform into another creature of its own (usually prohibited) type, but any creature can transform into a living one using Metamorphosis. You can both share your powers with Psicrystal and cast Self powers on it:
Share Powers (Su)
At the owner’s option, he can have any power (but not any psi-like ability) he manifests on himself also affect his psicrystal. The psicrystal must be within 5 feet of him at the time of the manifestation to receive the benefit. If the power has a duration other than instantaneous, it stops affecting the psicrystal if it moves farther than 5 feet away, and will not affect the psicrystal again, even if it returns to its owner before the duration expires.
Additionally, the owner can manifest a power with a target of “You” on his psicrystal (as a touch range power) instead of on himself. The owner and psicrystal cannot share powers if the powers normally do not affect creatures of the psicrystal’s type (construct).
- You can perform Fusion during any moment. But, as per power description:
Target: You and one touched willing creature of your type and your size or smaller
You and another willing, corporeal, living creature of the same or smaller size fuse into one being.
As such, unless both you AND your Psicrystal are of the same type, you can't Fuse. This practically means that in order to cast Fusion on your Psicrystal you have to either use Metamorphosis to both change into same type of creature (Humanoid, Aberration, Fey etc), or you both have to be of the same type in some other way (the easiest is to be Warforged)
- No. You can't do that.
Depending on what you want to accomplish, you may have means to do variations on this trick to achieve the results.
- If your goal is to be immune to Death effects and such, there's one simple solution. If you look at the list of creatures you can transform into, you'll see that it's more of a list of types you CAN'T transform into. Which means, that any type NOT on the list is a legit type for Metamorphosis. This includes Deathless type from Book of Exalted Deeds, page 157.
This type is basically what you get if you take Undead type and reverse negative/positive effects on it. It doesn't need to breathe, it's immune to poisons and death effects etc. It's as simple as Metamorphing into one. A Deathless is not a living being, so Psicrystal can't Polymorph into one and then fuse with you
- Warforged are Living Constructs from Monster Manual III, page 190. Either be a Warforged, or transform your Psicrystal into one. How does THAT help?
Since Warforged are living constructs, they are not immune to several types of effects, including Mind-Affecting ones. That would open your Warforged Psicrystal to the possibility of using Mind Switch on it, which makes the process as such:
Metamorphosis, on you and your Psicrystal. You into whatever you want, Psicrystal into a Warforged.
Mind Switch on your Psicrystal, make it fail its save. As such, it's a living being now, and can be targeted by Fusion power. Your own type doesn't matter
(optional) Another Metanorphosis into a more potent and less vulnerable Construct form as per your new (temporary) typing
- If you ARE Warforged, check with your DM if Warforged are a legit target for Fusion power. If they are, transform into whatever construct monstrosity you want, and change Psicrystal into a Warforged, then Fuse. Would work even better.
Hope this helps. If you had something else in mind, I'll try to cook it up as well