"It's a Malibu 120-watt transformer and I have 120 watts worth of lighting on the system. He told me that it could be problematic to max out the wattage."

100% loading is almost always a bad idea.

Try to stay no higher than about 80%

"He also told me that lights should not be attached to the end of the line"

If you have the common vampire (simple clamp on) taps to the lights the line should be run a little longer.

Digging and aplsicing after the fact is not going to help much.

Just remove the last light but leave the cable the same length.

Water starts into the cable almost from the moment you bury it, especially from a cut end.
Making it run a few inches to the first fixture gives you more time for corrosion to cause problems.

Thanks for the reply, brickeyee.

>100% loading is almost always a bad idea.I've ordered a 150-watt replacement to give myself a little headroom.

>If you have the common vampire (simple clamp on) taps to the lights the line should be run a little longer.I've found that the convenience in installing those type of connectors are not worth the sacrifice in reliability, so I have not used them in this setup.

>Just remove the last light but leave the cable the same length.Hmmm, not really an option in this case. That would leave a big dark pocket right next to the patio. Would it be worth splicing on the extra length if I carefully sealed the connections with a silicone-filled wire cap?

>Water starts into the cable almost from the moment you bury it, especially from a cut end.
Making it run a few inches to the first fixture gives you more time for corrosion to cause problems.Thank you for shedding some light on why the extra length is advisable. Didn't understand it until you spelled it out.

" Would it be worth splicing on the extra length if I carefully sealed the connections with a silicone-filled wire cap? "

It is still not waterproof enough.

Thanks brickeye. I appreciate your input.