Sorry, seems I'm not allowed to edit my post...
The solution is found in the T&Cs:
- Yes, you earn points and get benefits when booking through travel agents:
Points will be awarded for Qualifying Room Rates booked through IHG central reservation offices, IHG web sites, travel agents or directly at the hotel.
- No points for steeply discounted rates (
non-qualifying rates), such as Priceline, due to the well-known sentence in the T&Cs that is sometimes a pain...
Eligible Charges Worldwide include the following Qualifying Room Rates paid for hotel room nights: including the hotel’s Advanced Purchase Rate, Best Flexible Rate, worldwide sales negotiated rate, national/regional/local government rate and specified leisure rates as confirmed by IHG's HOLIDEX® Plus reservation systems [...]
In addition, in North, South and Central America and the Caribbean, points may be collected on locally negotiated rates if these rates are discounted less than 30%.
There is no rule saying "you must book through IHG to receive points and benefits" (unlike at Hyatt, where you will get benefits, but no points).