Boy? Goodness... Anyway... to Utopiasearchers, it is my personal opinion that frankt is actually not a traveller, rather he is doing self promotion for his business in Madagascar. I find tour operators who are sneaky like that, pretending to be travellers on the forums, really kind of untrustworthy.
In answer to your questions, please consider flying to Maroantsetra and organising a car hire up there. If any of the other contributors who know that area better are around, perhaps they could give you clear advice but as far as I know, the answer to your questions are:
- Yes it is easy to organise car hire in Tana. There are private hires (which I personally tend to avoid after some problems in the past) and there are plenty of professional operators. Please be aware of the "return to base" rule that exists in most of Madagascar. If you hire a car in Tana to go somewhere else, you have to pay for the hire of the car and the fuel for the return trip to Tana too, whether you return with the car or not. So a three day hire to go to Tulear is in fact a five day hire (plus fuel), to allow time for the car to return to Tana.
- No, I don't think you can drive to Marojejy from Tana. The road north of Toamasina (Tamatave) is not good, as far as I know. I may be wrong, some notoriously bad roads were fixed in recent years.
- You need to decide where you are going to hire your car from first, then get some recommendations for drivers local to that area.
And just as an aside, I guess you already know but March is the height of the wet season it can be very wet all over Madagascar at that time. You should consider your routes and means of travel carefully and if you want to go by road long distances, then allow plenty of time, don't try to stick to tight schedules, be flexible. Sometimes, you can wait days for a river to become passable, as just one example of what can happen.