The numbers that con adds at lower levels are naturally lower but the %age of your health is the same. Just like health scales over time, the damage you take will scale over time too. An extra 15 health by level 5(12con vs 18 con, or 14con and tough) may not seem like much but that's likely 2-3 full attacks from the monsters you're fighting. That's a lot.
As far as I can tell the benefits for paladins from charisma are that if your are real close you can add it to your saves, spell save dcs, and some channel divinities. A +2 is plenty good for that until mid-late game imo :/
As for twf vs Polearms;
Polearms are a great option but really only shine if you have the feat, so any time before that and they are just ok. It leaves you with an even lower ac vs using a shield because unlike twf, the feat doesn't add ac so your max is 19 with plate instead of 20, not huge but it's there. With the feat your damage is exactly the same on average with the polearm. Assuming all 3 attacks hit at level 11(18 str, with divine favor);
Polearm
(1d10+1d8+1d4+4)*2=33avg
1d8+2d4+4=13.5avg
total=46.5avg
TWF
(2d8+1d4+4)*3=46.5
So really the downside to twf is that you need to dip into fighter. And the downsides to polearm is that it limits your weapon choice considerably(from any 1h weapon to 3 specific ones), and lowers your ac by up to 2(1 if you take defensive or dip into fighter). Personally I think twf wins this as magical 1h weapons are, by the tables in the dmg, more common than polearms. And some of the spiciest ones aren't even available as polearms(Flametongue, Vorpal, etc.)