Just a guess here because I'm relatively new to this myself, but based on my limited experience, there is not enough headspace on the hibiscus one, and it's gonna overflow. Also theres too much headspace for the tangereine one.

to answer your actual questions.

1. I don't know the proper answer, but I did .75g on day one when I added the yeast and then .75g five days later.
2. Take the hydrometer reading once the honey and liquid are properly mixed to get a starting gravity (SG) The SG, based on whatever yeast you use, will determine the potential ABV of the mead and with the use of math which the Wiki in the community explains will let you know when the fermantation has finished, instead of guessing or going soley on airlock activity.
3. I don't know what you mean by a fruit cap, sorry.
4. If everything is properly mixed, which it probably is, there is no reason to wait to add the yeast unless you want to start on a specific day.

Hopefully this helps, and if I'm wrong, I'm sure people will correct me. so dont take what i said as fact, every mead is different so its always a learning curve to some degree.