Fastening leaning post

[Sleeto]

I am installing a new leaning post with circular bases. I can thru bolt the screws on the inside half of each base, but I cannot reach underneath to bolt the outside half because they are on the other side of (or over) the stringers. I assume I will have to use some type of self tapping screw. Is this correct way to do this?

What kind sealant should I use? If I were to use 5200, will I be able to remove the screws if need be?

Thanks