I do not think anyone should be able to summon villagers. They are easy enough to obtain as is, and it should be a challenge to gather them.
As for the butchers, there are towns with dozens, and some with even hundreds of villagers. They cause lag, plain and simple. It is rare that animal butchers happen, and I honestly believe that villagers should to part of it.
For starters, this creates a larger market for gold, for alchemists, and the various items required to refresh trades. Second, this helps prevent merchants from camping specific trades, and prevents monopolies.
I'm mixed about this suggestion. I believe that allowing villagers to be butchered would only favor the bigger and more established towns (as they would be able to recover faster from an across-the-board villager wipe), and thus leading to a disparity between those with higher resources v. those with limited resources. I guess if a merchant is part of the "1%" as far as the town they are part in, then this would be a good idea; for the other 99%, maybe not as good of an idea.
If a town and/or region is experiencing lag as a result of their "hundreds of villagers," then I suggest that the head of such a town put a cap on the number of villagers that their town has (in the interest of a lag-free experience for their members). There is no true need for more than one "perfect" librarian, farmer, butcher, and blacksmith; and if someone has 96 priests, well, if those priests aren't giving the results the merchant wants, then kill em off and get some new ones in.
Regarding the concern of camping specific trades, MC already has a randomized chance of any trade being upgraded, or in the event of a priest's enchantment trades, changed to a different enchantment trade. If a merchant makes the trade enough times, it will shift on its own behest as an element of randomness luck (or misfortune). It may take some time for some trades to shift, but in essence, there is no guarantee of a "camped specific trade."
As far as providing a market for gold, alchemists, and other items, I am unaware of a trade that needs an alchemist's expertise; gold is without value and will remain without value until armor durability increases for it (merchants won't have much need for it when there are other stronger trades to get emeralds with); and the items necessary to facilitate the other trades aren't hard to acquire (except perhaps for a deep-into-the-villager-trades cooked fish trade).
So I guess my argument is more of a federalist argument: leave the lag issue to the individual towns (i.e. the states) to deal with regarding their villager population rather than requiring the admins (i.e. the feds) to do a periodic butcher of the villagers. I'm no expert on lag, but I will venture a guess that there are other mem leaks that account for server lag superior to the few hundred villagers that are presently in Haven worldwide.
-yav