So from what i understand of items, bondstar is just the bonus you get from actually using a yarimon instead of feeding exp, but condense into an item. So you can feed level a yarimon up to max, give it bond star and it should be similar result to if you use that mon the whole time.
What exactly is Soul boost? I understand it's a stat boost to make weaker mon stronger (I remember they introduce it as something to let "cute" non-evo stay relevant), but how does it work? Is it a flat increase or scaling? If feed to a tier 1 yarimon and it soul up later, is the bonus lost? is it something that need to be feed early?
About bond star, yes. It brings the bond level to 10 istantly rather than defeating 100 mons. (each star in the info is 10 opposing yarimon defeated and it's for the whole party...meaning even if you use only your strongest yarimon, the other 2 get the +1)
About the increase, it depends on the level. For example, at lvl50 10 bond stars give a +10hp/+7atk/+7def/+9spe, while at lvl100 it's doubled so +20/14/14/18 (from a testing I did on moostar)
Soul boost is somewhat the same, meaning that based on the level, a soul boosted mon get +[value] in all stats compared to the non boosted mon. I didn't test if for lower tier mon it's stronger than its final tier, but I guess it kinda should be.
And no, if you soul up, you don't lose the bonus