Dear Mr. T:

You wrote:

Can one use:

 1-Hadn’t he opened the door, I wouldn’t have seen him.

instead of:

2-Had he not opened the door, I wouldn’t have seen him.

Can one use:

3-Weren’t he such a fool, he would have accepted your offer.

instead of:

4-Were he not such a fool, he would have accepted your offer.

All four are technically OK, but in normal speech only #2 or #4 would be used. In most cases when the contraction is used, the listener is assuming the speaker is asking a question. We would probably hear: “Hadn’t he opened the door? I wouldn’t have seen him.”

#3 might be a bit more clear because it is using the subjunctive which is rarely, if ever, interrogative, but it could still cause some confusion.