It's pretty easy to jam as it's just radio waves. Increase the noise on the channel and the chirps of your msg don't get heard. That said there are some options to vary the channel as a group, and jamming a broad and robust mesh completely vs an area of nodes is a bit harder.
Trackable as in traceable? You mean finding your node location? By default not overly difficult but again, can be set up to make it hard to find you.
Yes, which is why I said it wouldn't be overly difficult, especially with access to military equipment. TDOA could fairly easily pinpoint a signal location. Meshtastic is generally chatty but you can do stuff like reduce transmissions, and limit the amount of hops your msgs can make etc. or even if you knew who you were sending to, make it directional so it's harder to hear you. That said with the right tech, actively looking for nodes and listening to a chatty mesh radio it wouldn't be hard.