1) If it moves 70 kph it will move 700 km after 10 hours... with a speed of 70 kph... not 700. The speed in a forerst depends on several factors. How thick is the flora, does it consist of dense trees that cant be simply drove over, has the path been cleared by pioneers are there any obstacles like swamps etc. Actually the Chimera is rather slow but sturdy. With a dozer blade it should have have that much of a problem. Even less if a tank is ahead to clear the path so take a leman russ movements and reduce it by some kph. If they are on their own I'd say around 40 kph.
2) As long as you as a GM need it. You can construct your campaign and if you want them to go dry say they have to take a very long route, if not make it a shorter route. There is no value of knowing how much fuel they carry if you can adjust the path they travel however you want it to be, Though keep in mind that a chimera will always leave its base for a mission with enough fuel to reach the target and go home with some spare fuel in addition. Also your operators should have an Idea what their chimera burns so if the guys of the munitorium issued to less fuel he should notice it. At last thats its for the normal situation.
3) The tabletop has that rule for you mostly attack with a swarm and need to apply one modifier for all dices of said swarm. In ow on the other side you controll one guy, not a swarm of up to 20 or more compatants. Standing in front of a Leman Russ, declaring that you want to hit its rear armor while you chunk your bayonett on its front would be stupid. If you want to swarm it, you have to actually swarm it, meaning that one of your friends has to go on its back. Also the rulebook can not state all rules you might need and it is up to you as a GM to fill the gaps. If you think it is okay that after your weapon specialist climbed the tank with a climbing test if necessary that the hatch would have less AP than the overall top armor, so be it. If you accept that the main cannon is damaged without the necessary result after one soldier charged the tank and managed to throw a krak grenade into the main cannons barrel, so be it.
And for the weapon choices for close combat the rulebook actually states that you have to take melee weapons or pistols. Note that you are never in melee combat with a Tank unless you want it to be. You can stand right infront of a tank without being in melee combat because he can not engage you. It would be a different thing if you stood against a dreadnought and these things are even so fast that even in the tabletop nobody can engage their back armour if in fact 20 models stand right around them. So might consider melee vehicles to always get hit in the front too.