Get a better chancellor - bit of an exploit: characters who aren't in a line of succession will accept a matrimonial marriage proposal from any courtier of yours, and move to your court. Use the character browser to find a good chancellor.
You can certainly marry into claims and get them onto your heir (check that they are inheritable).
You can press claims on behalf of your courtiers. And the claimants button for a title will give you a list of them, and mark those who will accept an invitation to your court. They'll get the title in question, but I think as long as the title your are pressing is lower in rank than yours, they will be your vassal.
Once you have 50% of a duchy you can create/usurp the duke title, which gives you a ducal claim over the rest of the de jure duchy. This will vassalise the counts in question, although if the county is owned by another duke, you'll get it in your demesne.
Invasions really aren't very difficult to get. Some characters just seem to grow vast amounts of piety. It's a little odd to pick off 2 province duchies in papally sponsored invasions, but it works

Actually this was in the modded demo, a lot of things have been toned down from there and it may not be as easy.