I prefer stranded for cables connections that will see a lot of flexing, but if you use connection boxes that pretty much leave the cable stable, solid conductor should be fine and last for years with no problems. I've never had a problem with EMI on my hill, and I work in an area with a lot of stray signals floating around. However, if the cable is going to be close to things like transformers or electric motors (chairlifts) or runs with a lot of powerlines for snowmaking buried in close proximity, a drain wire might not be a bad idea if you can't attach a ground wire to the cable shield. Remember to only ground one end of the cable.
I use a 6 pair, 24AWG, shielded, duct-filled, and gopher-proof direct burial wire. The local ski areas lucked out and got a bunch of wire donated years ago when they put in the Alaska pipeline, and this is what we ended up with. I would have preferred more pairs, but who can argue with free? This year I found my cable had floated to the top (from 12"-16" down) and gotten mauled by the fall hill mowing. Repeated freeze/thaw cycles and the high ground water content in that area contributed to the movement over the course of 15 years. I dropped in a pole and box nearby and will be burying more cable to cover what I lost. Lesson learned? Order more cable than you immediately need. You never know what is going to happen.
I think the conduit might be overkill, but if you can swing it cost-wise, I say go for it.
Since 99% of the time I'm running single course races, I connect both wires of each pair together for reliability. That way I have 1 pair (actually 4 wires) for start A, 1 pair for start B, and a spare for communications or backup. All wires run to terminal blocks at the top and bottom, to save time if I need to make changes and to make troubleshooting easier. From there I run to Pomona banana plug connections at the start and in the timing shack, so I can cross-connect among pairs as needed. We used to use multipin connectors, but that was much less flexible and practical. I can't emphasize enough the need to leave "maintenance loops" at either end of the cable so when the exposed wire corrodes you will have spare wire to strip. Write down your wire assignments (i.e. Orange wire for start A, Orange/White for Start A return) in several places. I usually will tuck a laminated wire list in my connection boxes so I'm not freezing my fingers off trying to figure out wiring assignments when it's -10F and blowing. Since my start is not covered, I used weatherproof covers over the start boxes to keep the jacks clean of snow and ice.