1. There is no proof regime used chemical weapons.
No. But "proof" that will satisfy everyone is very hard to come by.
2. Why would regime use chemical weapons ? They're "winning" already..
I think you've answered your own question there with your use of quotes. The regime are looking at the thin end of a potentially multi-decade civil war. There's no prospect of a decisive victory in the forseeable future.
3. Why would regime use chemical weapons ? The rulling party there is NOT stupid, whatever you may think of them. You think they wouldn't know it would come to this (worldwide condemnation) ?
The regime has been torturing kids to death for years with little comeback. It has already achieved worldwide condemnation. The Assad family doesn't care. The only risk to them is military intervention by a superior power but after 100k+ deaths and years of atrocities they don't believe anyone will intervene.
They were quite good friends of many Western countries until quite recently. The US used to give them prisoners to "interrogate". Even after years of brutal repression with much video evidence there are plenty of people on the internet who believe Assad and friends are the good guys.
4. The ruling party have majority of peoples votes. Why would they undermine that ?
Elections in Syria are meaningless charades, of minor use to the regime as a rubber-stamp only.
5. Why would they allow UN to come inspect the site then shoot at them ?
It doesn't seem to be in either the rebel's or the regime's interests to shoot at the UN. It seems likely that this was an error by one group or another.
It seems entirely plausible and in character for the regime to use chemical weapons. They have the stocks and a history of being willing to use extreme brutality.
The idea that the rebels have launched the attack in an attempt to blame the regime is somewhat plausible. They are desperate enough and some of their elements seem to be capable of acts of harshness approaching that of the regime. However the attacks were in rebel-held areas and there is no evidence of the rebels gaining any chemical weapons.
I think it's rather more likely that the regime made the attacks based on past history and circumstantial evidence, but we don't have very much in the way of proof either way.