I can only vouch for the Dartford > Ramsgate section but that will be fine on 23s. The off road section is only short anyway so even if you did want to walk it it wouldn't be a drama.
On your route which section is off road? I've done what I thought was the John Mack approved route to Margate which was all on road. Isn't this the same?
After Rainham there's a private road that ends in a farmyard then a short section of bridleway before coming out on a road on the far side. If it's an issue it's easy enough to route round by going into Upchurch.
I can only vouch for the Dartford > Ramsgate section but that will be fine on 23s. The off road section is only short anyway so even if you did want to walk it it wouldn't be a drama.