And walking at my left side; she's decent at it, but she sorta does this "speed up, slow down" thing -- she should know where I want her to walk by now, but maybe I'm doing it wrong; or maybe I'm expecting too much from her (she's 20 weeks old)...eventually I would like to have her walking at a heel.
Yep, expecting way too much.
At her age she may have some idea of where you want her to be, but teaching "heel" is a process that can take some time, and she still doesn't have an adult attention span so I'm sure she's easily distracted. I literally walked hundreds of miles teaching Halo leash skills. I get compliments all the time now on how nicely she walks with me and how attentive she is, but that did not happen overnight. I would in no way expect a 20 week old puppy to REALLY understand heel. If Minna is doing pretty well on a loose leash most of the time, consider that a victory but keep working on it.
I agree with using a clicker and treats to mark and reward for generalizing "leave it" to other things and other situations. Basically you'd use the same method that you did with food and toys, either in your hand or on the ground. Either body block her from getting them or work with her on leash, but do not physically pull her back, just use the leash to hold her in place and then mark the second that she turns away from whatever it is and back to you. This is a great exercise for teaching impulse control, basically a default leave it:
I like to teach puppies "watch" (look at me), "find it" (look for a treat on the floor), and "touch" (nose bump to my palm), these are the trinity of attention directing exercises that are handy foundation behaviors that can be built upon later.
Rather than make a flirt pole, I bought this toy: Chase-It Pet Products, LLC
Something else you can do with her is mat work, teaching her to go lay down on a mat. I've combined mat work with a flirt pole, where the reward for staying on the mat is to be released to play with the toy, and then the dog gets sent back to the mat.