They’re correct, but the first one sounds awkward. The other sentences are good alternatives or another is There were twice as many accidents this week as last week. This changes your structure but keeps the twice.

Hi, Your sentences are correct. No, you cannot use "on" instead of "since". - You use since when you are mentioning a time or event in the past and indicating that a situation has continued from then until now. - preposition on: 1—in contact with and supported by Put the books on the table. 2—used to indicate means of being carried He rode on a tractor. 3—used to indicate the location of something Both bedrooms are on the top floor. 4—used to indicate the focus of a certain action He went down on his stomach.