1) What makes emission in thermals is exhaust system not the engine. In this regard Merkava is not different from T-90 which also has exhaust at side.
2) Merkava is deigned to fight the other tanks in hull down position. So there is no point to increase front hull armor against the APFSDS. Much better to save the weight for side, bottom and top protection.
3) L44 is more than enough against all Syrian and Egyptian tanks. Increasing barrel length will only decrease mobility and probability to hit on move.
4) Turret ammo or Merkava DOES have blow up panels. Hull ammo does not, but no other tank beside the Abrams have it. Merkava rounds are stored in fireproof containers in rear. Thus Merkava and Abrams are two best tanks in regards of ammo storage.
5) At 3:00 u can see that round penetrated the lower hull but did not ignite the fuel tanks. Beside their ignition does not endanger the crew at all.