The reason I ask is that its not clear if you need three Extra Spells (or Slots), or six. That is, will one 'bundle' of three Extra Spells/Slots cover both your Arcane and Divine spellcasting, or will you need two bundles?
Since Extra Spell/Slot were never designed to be used this way, I don't think there's a clear answer, I'm just looking for a bit of a discussion on it.
From the discussions I had with people on the topic I think it was always considered to be only 1 'bundle'. You can think of Extra Slot as adding to the table of what spells you get, making it apply to both your divine and arcane. But as you say those feats don't hold up very well in stressful situations.
Really the only reason you can get higher level casting is that Extra Slot is not worded very well, it does not specifically exclude from using one class (or ability for planar touchstone) to gain a new slot for another. It also only has a requirement of Caster Level 3rd so RAW as long as you have a caster level of 4 from somewhere you can get around the "You can’t use any abilities gained from your aptitude focus [...] to qualify for a feat, prestige class, or other option." clause.
IMHO the Extra Spell trick with Chameleons is 100% legit, and not really that broken with the added benefit of no arguments on if you can take a spell from outside your class list.
Sanctum Spell and Planar Touchstone however can get up to 10th level spells, opening up 7-9 with Extra Spell.
Yeh, my slight pet peeve with Planar touchstone is that from the wording of it. You have the domain spells as if you prepared them but you never actually do, so technically you cannot prepare any metamagic on the spells from planar touchstone. Now if you could get a Sudden Sanctum feat that would be a different story.
On a slightly different note I did port the Chameleon Handbook over to these boards (check out my signature) and I was once of the first people to do so, sadly it seems to be buried under all the other ones