Jogamel, it’s a challenging puzzle. The key to take from the preamble is the last sentence. All across answers must be modified in a consistent way before entry. So you can’t necessarily rely on any of your crossers when solving the down clues.
The good news is the down clues are entered normally. So when you have enough in you might get a good idea of what’s going on.
For 10dn, the wordplay is a 4 letter word for an extension to a ramp, inside a 3 letter support, all 7 being contained by a 3 letter word for as well as