GAPPY HEDGE NEEDS ATTENTION - advice needed please

Hello I am new to the forum and I am looking for advice please as i have a problematic hedge!! We have a long hedge at the front of our house (approx 18m long / 2.5m high) Mainly hazel, a bit of holly and lots of ivy….

About 4 years ago I cleared out the ivy from half the hedge and this year it looks really thick (hazel) The other half however is a mess of rotten trunks (hazel) and ivy with a bit of hazel in between. I started to clear out the ivy in one part of it and have created huge holes in the hedge. I really want to plug the holes with privet or some other hardy quick growing hedging plants however my biggest issue is that I cannot dig a hole in the gaps to put the plants in!!!! there are so many roots and the hedge itself it raised off the ground 2 ft (there was a raised bed in front of it which we took out so there is an area of sloping ground from the base of the hedge to the lawn - separate issue as can't get much to grow on this!!)

anyway, I digree. Please any advise on how to plant within the holes. The most I am digging in is about 15-20cm, i fear that I will damage roots if I try and use anything else create the holes

Please can anyone give any advice on how I can do this
also i have a very gangly hazel tree within the hedge with a couple of branches coming from a single stem. If i cut this down to about 3 ft will it thicken up or not?

thanks