Yes, the charge current will drop off as the battery becomes nearly charged.

Lithium batteries are charged using a process called constant-current constant voltage

What this means is that the charger will apply only enough voltage to produce the maximum charge current you have selected until such time as that voltage reaches 4.2V per cell (that's 8.4V for a 2S pack, 12.6V for a 3S, etc). It will not increase the voltage beyond this point so, as the battery continues to charge, the current will automatically begin to reduce -- until, when it reaches some pre-determined minimum current (normally less than 100mA), the charger will say "full".

As for the the number of amps it'll suck out of your car battery...

Well think in watts again.

If you're charging at the full 50W that your charger can handle then it will be drawing 50W/13.6V (assuming you have a fully-charged car battery that is putting out 13.6V) which is about 3.7A. That's even if you're charging a 6S pack at 2A -- which is also about 50W.

In order to allow you to charge batteries that are a higher voltage than your car battery, the charger includes a voltage boost circuit that converts the 12V of your car battery into as much as 30 or more volts (depending on the charger). However, since it's all about watts, 1A of current at 30V is 30W and 30W at 12V is 2.5A so the charger will draw more current than it appears to put into the battery.

So, to figure out the maximum current your charger will suck from your car, find out how many watts it is and divide by 12. That assumes you're running the charger at full wattage and that your car batter isn't fully charged.

A 50W charger will suck a maximum of around 4A

A 300W charger would suck a max of around 25A

and you can calculate the rest for yourself.