Watched a little bit of your video Dobby. Which included a bit near the beginning where you moan about the magic mary tucking under on turns.
What size, compound and casing Magic Mary are you running upfront?
and what pressure do you run it at?
I've only ever found a MM to tuck under (If we're talking about the same thing) if it's run too low pressure on hardpack (Which is what you're riding in the vid). Basically from too little sidewall support.
too soft (unsupportive) or slow (rebound) fork settings can cause a similar issue. But if you've only changed the tyre it's unlikley to be your fork.
as for the lad kicking your arse. From the short section I watched your line choice is poor and you're not setting up or positioning your entry to most corners well. By entering the corner in the wrong position you're losing speed in every corner. Get out of the sheep lines. Set up wider entering corners when you can and brake less before entry so as to exit each corner with more momentum. ie. FREE speed