I have to disagree - in my experience even amateur rehearsals should and generally do start the moment they are scheduled to and this tends to be enforced. Several reasons for this, not least bein paid by the hour and rooms being booked by the hour, but also that if the conductor or administrators show a relaxed attitude to timekeeping it can demonstrate a realaxed attitude to the business of making quality music. If the players learn that they can roll up ten minutes late, they will start to arrive later and later and eat more and more into rehearsal time.
If a rehearsal is scheduled for 2pm, the tuning A is taken at 1.57pm and the downbeat is two. Any director worth their salt will have made a schedule for what needs to be looked at in what order at what time, allowing extra for any delays or unforseen problems.
Also, it is not really the composer's responsibility to be in charge of logistics. If the venue has staff, they should do any setting up that falls under their jurisdiction. If not, appoint a stage manager. Musicians should be responsible for bringing their own instruments and accessories (except if the venue has music stands or percussion of their own). All the composer should have to do is write the music and possibly print and prepare the parts.
If there is more than one rehearsal, the first one should start with a read-through. Players like to just get through a work to gauge its length and character, and unless it is very complex, chances are most competent players will sight-read to the end with few or no restarts. After this you can start to rehearse things. One of the comductors at my university simply didn't understand how much he would annoy players by continually stopping and going into tiny details before we had even had a chance to get a feel for the piece. Some directors like to play through the whole piece again in the last rehearsal, but others do not and just go over selected passages before the final performance.
Finally, I always try and get the music to players in advance, and if they are likely to have a poor work ethic I accompany this with a message suggesting what a douche they're going to look like on concert day when they're the only one who hasn't practiced.