For stutter and freezing problem you could try the k-lite codec pack. WMP will pick up the codecs and use it for playback. It could go alot smoother for decoding on GPU or use more CPU but not recommended. Using Handbrake (explained in the xibo manual) will give you the best practice in how to re-encode the MP4 for best playback.
For the player delayed start, there is no simple solution which involves changes or behavior of wmp. You could use the transition part with a fade so the player starts earlier with opening wmp and load your MP4.
I would recommend to use the Xibo Android DSCS9X. We have a few customer still running on Windows, they were testing it for us and kept the device. We were also using clips but after a few weeks we changed it to a Youtube playlist. The same clips they want to play in the showroom also appears on Youtube. Another customer is using the interactive functionality because we integrated an alarm system with a panicbutton. It is easier to tell xibo on windows to trigger a layer without internet. Our solutions keeps working even if there is no connection to the CMS/XMR.