Can a Prophecies character...

Use the sword models introduced from the expansions, I only have Prophecies.

Thanks in advance

Yes, as long as it's not customized; can use any weapon from any campaign regardless or origin (proviso: you meet the requirements to yield it; i.e. tactics/sword/scythe/etc)

Yes, apparently the only content you can access from other games if you don't own them are basically items that someone can bring to you in trade from that said game. Everything else requires you to add the key to your account.

Keep in mind you won't be able to obtain them yourself via. drop; you'll need to buy them off someone who has that campaign.

Just get a friend with all the campaigns to buy it for you :]