It takes young puppies a LONG time to get used to walking on a leash nicely. I had exactly the same problem with Cey when he was about Bear's age - - he had to stop and sniff everything, and, if I tried to force him to walk, he would just plant his little butt down and refuse!
Maybe there is better training advice out there but here is what I did (and, Cey has no trouble walking on a leash now, though I do let him stop and sniff occasionally): I just let him do what he wanted to do, and when he did what *I* wanted him to do (walk forward), he would get a treat. Positive reinforcement, right?
. Seriously, even with that, it seemed to take for freaking ever for him to finally walk nicely on the leash... but now, looking back, he was walking pretty nicely in general just a couple of months after I started the positive reinforcements. Now, he doesn't get treats for walking nicely but does get to stop and sniff occasionally and he is pretty content with that.
I seriously think that it just takes patience. The worst thing you can do is make a walk a negative experience for your little girl, so, just remember that. And, with consistency and training, before you know it, she will outgrow the 'plant the butt down' phase (at least for the most part haha!).
As for the harness, I would just try to get her used to it during other times than walking. Put it on her at other random times during the day, just to get her used to it. A harness REALLY is better, especially when they get older and bolder, as it doesn't hurt their little necks if and when they pull.
Don't know what to tell you about the yelping... sounds like she might do that when she is surprised maybe. Others more experienced than I will, I am sure, chime in but it seems to me that as long as she isn't acting aggressive or fearful, the yelping is nothing to worry about... and if she is acting fearful along with yelping, there are lots of things that you can do to help both you and her with that (again, I will let more experienced people chime in about that!)
Love your Bear's colorings by the way! She is a beauty!