##Arcane Spell Failure
Arcane Spell Failure only applies to Arcane spells that have Somatic components, yes. Still Spell is a (very expensive) way around it, for example.
There are far, far cheaper ways. A +1 twilight1 mithral2 feycraft3 githcraft4 full-plate with thistle-down padding5 has a 0% Arcane Spell Failure for just money, specifically 15,600 gp. Saving yourself a feat and a +1-spell-level cost on most of your spells is easily worth that much.
Using mage armor or a Chain Shirt (+1 twilight mithral, mithral fey-or-githcraft with thistledown, etc) of some kind would be much wiser, though, since it is a lot of money. AC is not that valuable for most spellcasters. A dedicated gish probably wants levels in prestige classes that mitigate or ignore Arcane Spell Failure anyway.
1 Magic Item Compendium, reduces Arcane Spell Failure by 10%. +1 equivalent, magic.
2 SRD, reduces Arcane Spell Failure by 10% among other things. 9,000 gp for Heavy Armor, mundane.
3 Dungeon Master’s Guide II, reduces Arcane Spell Failure by 5% among other things. 500 gp, mundane.
4 Dungeon Master’s Guide II, reduces Arcane Spell Failure by 5% among other things. 600 gp, mundane.
5 Races of the Wild, reduces Arcane Spell Failure by 5% among other things. 250 gp, mundane.
Combining feycraft and githcraft requires either a team effort by a fey armorer and a gith armorer, or some kind of weird fey-gith hybrid armorer. No rules explicitly detail how to combine these two, but no rule restricts them from being applied to the same armor either. If the combination is unavailable, a single level of Spellsword (Complete Warrior) can handle the remaining Arcane Spell Failure for relatively low cost.
##Mounted Spellcasting
This has nothing to do with gestures, and everything to do with not losing your focus on the spell. Still Spell has no affect on it.