Having played the demo so many times here’s a little more insight i can give:
Dodging Perfectly - For me i feel it works best if I’m moving/walking in the direction i want to dodge and then pressing R1 as soon as the zombie launches on me.
Downing Nemesis - Any explosion downs Nemesis. You have about 30 seconds before he recovers. Use this moment to your advantage. You can use granades (1 is enough) and you get two throughout the demo. Also, those red barrels lying around do the trick as well. Unlike Mr. X and more like the Alien in Alien Isolation it seems he is invisibly tethered to you which must be why he’s always around. You can use doors to slow him down a bit as he tries to fit through them. Lastly, there are certain areas where he won’t follow you so keep that in mind.
Getting the Sub Machine Gun - Let yourself die and don’t choose continue. Instead choose “Change game mode” which lowers the difficulty. Jill will now the weapon on her + more inventory slots. Also, run back the train area where you started to get SMG ammo. Oh and on Easy combining ammo yields better results.
Opening the safe - Look for the answer in the farmacy. Doesn’t matter which way you start the combination. It can be left, right, left or right, left, right despite the sign saying otherwise. I believe this was the same in RE 2 but i didn’t know this.
As for the toy bobbleheads you really have to look at a video guide. Some of them are so tricky to find you need to perform certain tasks before they appear.
Given these games are built for speedruning i decided to try it myself and after a couple tries i managed a time of 2:47 mins. Since all you really need is one key item, accomplishing this wasn’t difficult at all. I’m 100% sure someone out there did it in less than 2. lol