I've noticed that on some loks too, the coupler is lower than the coupler on the cars. Since the couplers have plastic shafts, there's not much to do about it...

What I do is to make sure that the loco coupler loop is the lowest one in the coupling. Mostly works, but not a really solid solution... To replace the first coupling on the first car with a Relex one is a bulletproof solution, but increases the distance between loco & cars a little bit...

If possible (provided that both the loco and first car are equipped with NEM pockets) a tip is to use another coupler as eg the Roco Universal short coupler.

To answer the questions...

1 - Yes
2 - No

The coupler loop head is a bit wider than the M ones, so in rare cases you might get buffer "intervention" in tight curves... They do however couple rather nicely with the M coupler too..

They couple much better than M* couplers and require less force.

Only drawback: Roco universal couplers cannot couple with loop and hook couplers (may disconnect in curves).

Back to topic: I have Märklin close couplers where the metal loop doesn't move freely. It may help to loosen the lids that hold the metal loop to the plastic part (if that's the problem).
Or replace the coupler.