Need help understanding CDAF and if a lens makes a difference

So I'm barely holding my head above water with what I know about SLR's in general. I need to take a class but demands on my time don't yet allow it (maybe an online course, still looking).

Daughter is a sophomore now and the season has just started. She's had a few high school volleyball games so far and just yesterday I decided to break out the K3 and Tamron 28-75. I usually start off by taking a shot in Auto to see how the camera reads the situation and adjust from there. It didn't seem to matter if my shutter speed was 100 or 500, ISO 800-12800 ( I'm uploading to Photobucket now so I'll try to share a couple of pics). I was struggling to get any photo that wasn't blurry until near the end of the event I switched to live view. Huge difference in keepers. I can't possibly take all shots in live view so my next thought was about my lens.

I bought the 28-75 a few years back because of all the reviews and other than my experiences with lousy lighting with indoor sports, it's done very well. In your opinion would another zoom lens make a difference? I just don't see anyone talking about this lens anymore and I don't seem to find anyone comparing it to anything newer. Are all offerings from the Pentax 16-50 to the new 16-85 or the Sigma 17-50, etc that much better? Or might you have any advice on how to get the K3 to focus as well using PDAF in this environment?

I'm willing to replace the 28-75 but I'll need help on which is the fastest to focus and which seems to do better doing so in low light.

In 70% of the school gyms we'll be in I can get fairly close to the action. I was sitting half way up the stands at this last event and tried to use 75mm to see if I could manage with the Sigma 85 1.4. Although I could, sometimes (most of the time actually I shot between 35-50). The next tourney I'll solely use the 50mm 1.8 and see how it goes.

Thanks

Drew

On another note, can anyone suggest a short, light weight monopod? I'd like to use it while sitting down in the stands then use it on the floor as well.