I said you couldn't do all the routes in a day from White City, finishing at Russell Square. Of course you can do as you suggest which adds in the links from White City to Russell Square before starting the express routes from there.
I think even starting & finishing at Russell Square & catch the 15.50, X26, is doable in less than a day, the last 607 arrives in White City around 10pm, quick hop to Hammersmith on the 72/220 then the 10, the 10 takes 40 minutes in the evening Hammersmith-Russell Square, so you'll be back in Russell Square 23.30-midnight.
I'm planning do a visit to London for October, so probably end up trying this out, although i might do it the other way in the morning peak if i don't do it in the afternoon peak,. My itinerary would be start & finish at Heathrow;
X26 Heathrow-West Croydon, get the 05.40 which arrives in West Croydon around 07.10
X68 West Croydon-Russell Square, any bus from West Croydon until the last one at 08.30
then make my way to White City for the 607 then A10 to Heathrow, might go a different way to White City by bus only, rather than get the 10 then 72/220
The advantage of this way round, are the early morning X26s are quite fast traffic doesn't start to build up from 06.30-7am, & i'm not worrying over any delays to either X68 or X26 that might make me miss the last 607 from Uxbridge, only planning is getting to Heathrow for 05.00-05.30 on the night buses
Edit, starting on the X68 from West Croydon isn't a bad idea either.