Q1: I'm ambivalent, but no. The DM is already hugely underplayed but good players make use of Ensnare all the time, it's the redeeming factor of the hero to me. It already has to Psi Storm too to keep up in Exp and has no consume until L4 though, so I don't think that'd be for the best.
Q2: The extra building for such a small thing seems unnecessary, unless: I still don't see why you don't just have 12 sets of buildings and give them dependent upon hero chosen. That cuts down on a lot of unnecessary units. It also takes even less locations than it would with 6, if that's the problem. 4 locations, split the buildings into quarters and use give all owned by player 7/8/Neutral, and distribute them accordingly. (If you can't give from Neutral/Tan, it still takes 6 locations.)
For example: Mech's buildings are initially White, Spec Ops's are Yellow, and Medic's are Tan. If player picks Spec Ops, give all buildings owned by P8 @ location "top 3 heroes" to Player x. Sorry, tangent, but probably worth the change, saving space, units, and buildings.
Clans, I'm almost positive that it'd be worth it. It would allow you to be efficient at surrounding even without the speed upgrade, and with it you're almost assured of a surround on the slowly retreating enemy. It can be cast while burrowed, too. If it ends up not working out good enough, I'd be pretty surprised. My concern was actually that it'd be too powerful for a Summoner L2. It's also a defensive spell, it would let him escape from pursuit if cast well.
Re: Excalibur. The summoner does summon, but having progressive spawns keeps the Summoner as a no-risk hero. He stays in the base and uses his L1 spell to send lings into the field. Most players don't actually bring the summoner out of base at all, though better players take risks with Dark Swarm, Plague, and Consume. The new L2 idea would give the impetus for leaving the base, and the L3 support spell is already in place. This would allow the Summoner, like most of the other heroes, to weigh risk and reward and choose whether or not an attack is worthwhile. Such an attack poorly calculated could result in the Summoner's death, which in the current and in a progressive spawn style almost never happens.
None.