The 1999 KG limit is a paper limit not a safety based limit and I know of no private op Senecas which are flown to that figure.
Of course not. The Seneca V has a structural limit of 2400kg (IIRC) and will carry full fuel and four adults without any problem. But if you have your landing accident on one of those flights your insurer might have a valid point for letting you pay for everything yourself. That was one of the reasons why I didn't continue to fly that Seneca V (the other being the long downtime after it's landing accident during which I found something better to fly
).
If someone wanted my advice on what six-seater piston twin to buy, I would not recommend a Seneca unless he really wants a factory new plane (not much choice there...). For the price of a Seneca V you can find a good Cessna 340 (or 303 if you want systems similar to the Seneca) and have enough cash left for two engine overhauls, a glass-cockpit retrofit and ten years worth of airway fees. And no C 340 I know of was ever crashed on landing.