As far as I can work out, the Castle HSTs finish Saturday, but not all 3 diagrams need 175s immediately
Next week Looe branch is closed releasing a 150 which might be cover (either directly or indirectly) for a 175
Following week is Christmas week, when services are thinned anyway and school train strengthening is not required.
Full service including longer school trains doesn't run following week (New year) either.
The real crunch for 175s would have been when everyone goes back to work and school on Monday 5th January, except GWR have given themselves bit of extra breathing space with DMUs by suspending St Ives - Penzance services for 2 weeks.
As far as I can work out the first date when full DMU service is required in Devon and Cornwall is Monday 19th January. Hopefully by then a few more 175s will have been delivered.