Euron likely had his own fleet before he even set foot back on the Iron Islands - and probably had some plundered loot to help pay/build more - would be nice to have seen a couple of scenes showing this though. Even though he says there are 1,000 ships - it doesn't necessarily mean he has that amount - he does seems to like to 'talk the talk' as it were and may be exaggerating a little...
I don't think the 3 Sandsnakes would poison their blades all the time - they may carry or know of antidotes - but wouldn't want to risk accidentally cutting themselves or allies whilst looking after their weaponry... and they were all caught off-guard by a surprise attack - 2 of whom were seemingly drunk at the time.
For Cersei - we've been has shown has she manipulates people to get what she wants - and also her own shortcomings in the minutiae as well as lacking an understanding of the bigger picture (ie Jamie telling her she doesn't understand the severity of situation in the 1st ep of this season, her mistakes on the Small Council previous...) Leaders, Kings and Queens make mistakes in this show no matter who they are - how smart, noble, savvy, honest they appear to be... and Cersei is not great at actually leading - she excels in the journey to get to the power and the manipulation side of things...