1. On stacking limits in a two player game you’re correct - 4 Units total. Only 2 units per player, and only 1 of the 4 can be a vehicle (whether it’s a Jeep or a King Tiger doesn’t matter). However in multiplayer games it may work a bit differently.
2. Units never block line of sight.
3. Hills are basically no different from forests - they don’t give you any extra vision on their own (though some units can benefit from them because of SAs, like the Nashorn); the only difference is vehicles have to make a movement roll into the forest. That’s it. AH/WotC kept the rules fairly simple so no extra LOS mechanics.
4. You can move into a hex with an enemy unit, but doing so provokes defencive fire, as GrimJesta states. However remember disrupted units can’t defencive fire either. Defencive Fire is provoked if a unit moves from an adjacent hex to another adjacent hex (note moving out of an adjacent hex does not provoke defencive fire). The defencive firing unit can decide which hex to make the attack in. Roll the attack normally, but if you would score 1+ hits you simply cause an immediate disruption instead, causing the provoking unit to stop in that hex and end its movement. If the hex the provoking unit is being attacked in provides cover, then the unit rolls; if successful then the provoking unit isn’t disrupted and continues on. Each unit can make one defencive fire attack per phase, so a standard tactic is to move in first with a less threatening unit to draw fire.
5. You can’t die from defencive fire unless it was caused by a flamethrower; the three 6’s effect still applies. Except for units which are immediately destroyed (death by flamethrower, or a Carro Armato going up) the effects of damaged/destroyed/etc aren’t applied until the casualty phase after both players have made their attacks.