When this happens, try moving them from clear terrain (plain grass and dirt) to something like a road. IIRC, Bittah knows why this happens but can't find an elegant way of fixing it. The same thing happens with the Stealth APC and GDI's APC.

Blue tiberium and one of the types of tiberium from meteors (which rarely gets to spread, but when it does it happens very quickly) will often explode if hit by explosive munitions (like the shot of a tank). The explosion causes the rest of the tiberium to explode, in a chain-reaction (which you noticed). It's quite hilarious to lead a group of enemy units onto a blue tiberium patch.

Making it explode reliably is difficult, but Nod artillery can usually do it very easily.

Generally weapons that have a somewhat wide damage radius such as the artillery work best for this, but alternatively fire/napalm weapons work quite well for igniting a chain reaction as well.

I don't understand your question... When a vehicle's health becomes yellow (meaning it dropped below 50%), a vehicle will start smoking and sparking for a while and after that while it'll stop smoking, but will only continue to spark. The same thing will happen again when the vehicle's health becomes red (meaning it dropped below 25%); it'll smoke and spark and after a while the smoke stops and it'll only spark.

Assuming you have Visual Details on High (even Normal could be enough, I'm not sure). To access the setting, open the Options menu while in game and go to Game Controls.