1) Depends on the LED lights driver circuit. Different brands electronics. May look like a different type load.
Not resistive as X10 two wire dimmer switches will not work with LED bulbs. As they get power through the load.
I have had SOME dimmable LED bulbs work fine with X10 Lamp Module. While others failed miserably.
2) The AC On or Off Sensing current and Local Control Sensing current in the older Appliance Modules. Can in some instances
Cause the LEDs to flicker or pulse when Off. The later CFL friendly appliance modules are much better.
The realy type wall switches also have the AC Load On or Off sensing circuit.
3) Depends on your RF distance from the SS13A to the TM751 and power line conditions from the TM751 to XPFM.
Many new electronic devices. Can cause noise or signal sucking of X10 power line signals.
Jeff has a great set of tutorials you may find interesting. http://jvde.us/x10_troubleshooting.htm
I have both an XTB-IIR repeater coupler and an XTBM X10 signal tester. Both are top notch products Jeff designed.