You can suppose any sentence you want in order to draw conclusions. To suppose a sentence does not mean that you are persuaded that it is true. And, of course, you can suppose conjunctions. After all, this is how reductio-proofs work. If, however, you insist to introduce a notion of supposition of "single thoughts" (suggested by your premise (4)), then you probably have to add one more premise to your argument with the strength of: A single thought is one which can be supposed (in the sense of the other premises) and of which every implication can be supposed. I.e. you would need to introduce a notion of supposition which is closed under implication. But then you would not need to suppose anything, because by supposing you would have to pre-suppose that you know all implications which your supposition has. So why bother to prove anything?