1:Why do you recommend Wizard20 instead of Rogue1/Wizard8/Rogue+1/Wizard+X for a Sun Elf?
Because you have to LR+1 to remove the first cleric level, which makes the build quite expensive. If you're going to spend that much on a DC wizard, it only makes sense to min/max that wizard's DCs. Splashing 2 rogue is definitely not min/maxing your DCs.
2: For a Pale Trapper, how important are summon spells and the Skeletal Knight?
3: Does Augment Summoning affect the Skeletal Knight? When is this feat generally worth it for a pure Wizard or Pale Trapper?
It does indeed affect the Skeletal Knight. It's only worth it on a human, first off, because they have an extra feat to burn. The pet is actually halfway decent up through around level 12 elites, plus in all cannith challenges.
4: What DC and spell penetration do I need to be at the levels of competitive and optimal in Epic Elites at 21 and 28?
For epic elites at level 21 you want at least high 50s DC, preferably 60; by 28 you'll want 66 or better. A pale trapper is simply not going to be able to reach that. For spell penetration, you'll want mid-50s (54 is a solid number to reach) for the low level epic elites. High level epic elites require no spell penetration at all, as far as I'm aware. (Low level epics are crawling with drow.)
5: For a negative energy-heavy build, why not put more points into Heal?
Skill points are tight; I put as many into heal as I could. Spellcraft is more important, IMO, but I could be swayed toward heal instead.
Yes, a sun elf must spend a heart to remove the first cleric level. That means your sun elf life should be your very last life. If you're focused on high-end content as a DC caster, the first thing you should focus on is getting completionist. After you get completionist you can consider tacking on additional wizard/fvs lives for spell pen, sorc/cleric lives for DCs, etc...6: Reincarnation-wise: I was considering maxing out my Human Sorcerer then using reincarnation (which flavor) to return me to 20 (preferably) as a Sun Elf Wizard. Which is the optimal reincarnation path for someone focused on high-end content? (It also helps I bought the Shadowfell Conspiracy deluxe edition.) If I do reincarnate into an iconic, must I still use a Lesser Heart of Wood to remove the first preassigned level?
Completionist isn't required, of course, but no single point of DC is specifically required. It's just that when all is said and done, getting the absolute maximum possible DC the game allows ends up with a functional DC caster that the endgame content is built around. Not having completionist puts you 1 DC below that if and only if you already have every other possible DC boost in the game.