1) When I go looking the first thing is, what can I get to with the slide in. Important for me is the frig, toilet, bed (DW likes to nap while traveling), microwave. Those models are out there it just takes a little work to find.
2) I would never open a slide part way, but that's just me. I feel you are asking for trouble by doing that. I know that the schwintek brand of slide states all over do not stop slide part way. If you do the motors will get out of sync and you will be in for big problem.
3) As for power pedestals an easy way is to cut a rope the length of travel the slide makes. Go out and check with the length of rope if you clear the power pedestal.
Making sure that all of the seals are in good shape and in the correct location will make a big difference in the water proof issue. Having a awning topper on the slide makes a big difference in keeping the top clean.
I have had two rigs with no plumbing on the slide and one rig with plumbing on the slide. I am in favor of no plumbing, propane, on the slide. I had no problem with it but I only had the rig a couple of years. I could just see things happening easier with moving pipes.
Out of all I think the most important is what can you get to with the slide in.
