Hmmm.... I'll take a stab at this:
Can this spawn be improved?: Certainly - anything "can" be improved.
for example, the champs insta kill any mellers with an EQ for 150 points.: Sounds like a tough fight - spend a little more time there and you'll figure out better tactics for working it.
There is no way to tell the progress of spawn: There is a way - just as a normal altar, there are "levels"... stand near the gate that crosses the road and watch for "barked" messages... level 1 has no message... level 2 says "I see invaders"... level 3 says "Kill the invaders"... and level 4(Final) says :Meraktus is here!"
The respawn rate grows to a crawl near the end": Venture out a little farther and lure more up.
There are no scrolls: The rewards for this one have always been minimal, but at least you can say "I did it!" We did it many times ages ago - when most believed it did not exist!
8 out of 9 runs produced no replica or deco.: Sounds like the UO RNG is running true to form... lol
Even though there is minimal reward, I still enjoy it as something different...