Glad you're enjoying the mod!
We recently changed the file structure for some of the resources, we forgot to update the entry for the assault bike to match, it'll be fixed in the next version. To fix for now, find the file Ruleset/extraSprites_40k.rul, open it and look for the lines
- type: BIKE.PCK
subX: 32
subY: 40
width: 256
height: 840
files:
0: Resources/bike/BIKE.png
Change the last line to
0: Resources/bike/ULTRAMARINE/BIKE.png
Which units are you talking about unlocking after research? New armors should appear in "We Can Now Manufacture" or "We Can Now Purchase" popups after the research is finished, same for HWP-type units.
The 3rd weapon slot for some craft is for specialized equipment that you can research and manufacture to improve the crafts' stats.
As for the last points, how familiar are you with the WH40k lore? Space Marines are meant to be far and away better than either Guardsmen or Battle Sisters, as the marines are superhuman soldiers genetically designed to be the most powerful warriors humanity has to offer. Gameplay-wise, the sisters still get power armor for only being a bit more expensive than marines, armor you don't have to research or manufacture. They're a way to get some early durable soldier when you don't have good armor or resources to make it, but they won't be as good on the offense as a marine. Guardsmen are meant to be played as overwhelming numbers, and get tools for that kind of strategy if you research Imperial Guard Operations instead of a space marine strategy.