You guys are doing too much with her single success in the FF era. She did just fine and when you look at the bigger picture she actually did pretty good. You said it yourself, Britney only gets Pop radio support which limits her sales, most pop girls would be selling around the same amount of singles as she did with the kind of airplay she gets. 
HIAM may have underperformed but saying it 'flopped miserably' is ludacris considering it still broke records and sold over 3 Million worldwide. A #1 hit can never be considered a flop - Period.
And TTWE didn't do 'decently' either, it actually had amazing longevity worldwide and sold VERY well considering the airplay it had. Both Rude Boy and What's My Name had over 150 Million AI and MUCH more promotion yet still sold less/around the same as TTWE, which had only 99 Million AI and of course barely any promo. Another example would be P!nk with Blow Me, a song that reached over 160 Million AI and will struggle to go 2x Platinum and outsell TTWE. 
TTWE also performed better globally than 'Circus' did (I think), the same goes for IWG and IYSA.  The problem is that you guys have your expectations set too high, she was never gonna score 4x or 5x platinum singles like Katy, etc. Britney started her career before the digital era even started and couple that with the fact that she doesn't promote explains her single sales. 
Now when it comes to the disastrous album sales, that's a whole other story.................
