1. Instant cast GB. This is after the damage buff. If you're talking about the recent change, then it didn't buff our damage, it reduced the damage penalty when attacking from further than 15 yalms and nothing else beyond that from my experience; I still get a 20-50% damage penalty if they're closer than the 15-yalms.
2. I'm not sure what to make of that first video. The SMN not finish their burst, and they ran into your sweetspot range. By all means he could've put up much more of a fight if they had hugged them the entire time.
3. Second video, same thing. If the DRG hadn't come in, it could have been either way. You had no purify so DoTs would've been a problem even after recoup. They were also staying near your sweet spot range rather than hugging you. Without the DRG assistance you would've probably died to the deathflare that would've soon followed and make for a loss in trade even if the SMN died afterwards because of DoT damage.
It's a wet noodle when you're in melee range, I'm sure you know this. My dogfights, when they do occur, is a mix bag. I have my times that I'm fighting a SMN that feels the need to stay in my sweet spot range, while others are perfectly aware of my damage penalty and stay to me. You could stun gun (though I'd rather save that for melee LBs or a group) and blank them, but it's still not a perfect setup as you'd like it to be since that sets up for purify.
When it comes to using rapid fire, you either use it to set up the burst, or use it to speed up the burst (the latter being my preferred if they're in a group since it minimizes the window of opportunity for them to get healed), and you only really need to get a 1-2-1 shot with reload in before it's ready to go, which can be done outside of GB.
The sprint +TP is not a huge investment when it comes to that fight, but I'm speaking in general, because unless the summoner is quite literally by themselves, there could be a melee right around the corner with weapon throw. A staple doesn't change the fact that it's still investing one of two skills that have a relatively long cooldown, and it can also be used for getting out of a unfavorable fight/situation.
I'm not saying MCH is weak; I play one myself and I regularly score kills on other summoners and battlefevers, but speaking from a general standpoint they're not quite as versatile or anti-situational as a summoner, and the 1v1 scenarios that we're on can go either way depending on the environment and circumstances behind it.