I completely agree with stbr's answer for the first sentence, that's how I would have said it (in case it gets deleted or edited, I copy it here):
Je veux dire, ce n'est pas (c'est pas) comme s'ils allaient me laisser partir
For the second one I would prefer to use a slightly more elegant sentence using "Non (pas) que" and avoiding the repetition of "pas":
Je suis heureux que tu aies fini ça, (non) (pas) que j'en doutais mais ...
Note that "non" and "pas" are between parenthesis because each could be omitted, but not both at the same time. Omitting one or the other changes slightly how formal the sentence is. Here's my ordering (based on my feeling) from least to most formal:
- Pas que j'en doutais
- Non pas que j'en doutais
- Non que j'en doutais