what brake lever do you have? Most levers now a days are split clamp so you wouldnt have to take off the grip... well anyways to fix your problem you will have to get a new bolt for your grip and drill out your old rounded bolt. It's hard to not damage the lock ons since they're so small. If you aren't very experienced you will most likely not be very successful so you might want to bring it to your bike shop and ask what they can do.

No offense, but your bikes are trash.

Nothing wrong with my bike mate

SICK BOIKE

If you ride in conditions that get stuff everywhere get locks ons. If you generally ride dry dirt/street/park you can run non lock ons without them slipping. I run lock ons on my fully and normal grips on my hardtail (ruffian lock ons are the best)

Generally for a DJ bike you want something light and agile so it's easier to control.

Oh wow that's quite few bikes. Take a look at shimano slx brakes. They have dropped in price compared the years before.

I am really not sure If I can help you here. If that is your bike on your profile that I see then I feel like money could be better spent, but in the end it is your own money and I hate to feel like I'm telling what someone should do with their own money. If I were you thought I would probably save up and then sell what you have and invest in a better base of a bike. Take note that a nice bmx will cost considerably less than a mtb of the same level.