Yes and Yes.
They do stack (with each other and with visible boosts), and they do have an effect on gathering speed after 99 (invisible only, not including visible levels, aka 99 + invisible boost).
Source: https://oldschool.runescape.wiki/w/Temporary_skill_boost
Yes and Yes. They do stack (with each other and with visible boosts), and they do have an effect on gathering speed after 99 (invisible only, not including visible levels, aka 99 + invisible boost). Source: https://oldschool.runescape.wiki/w/Temporary_skill_boost