I'm going to use an argument that MITD is a Juvenile version of Godzilla.
Minilla aka Minya aka Baby Godzilla aka Godzilla Jr. aka Godzooky.

1) Has a plausible explanation for the Escape
Minilla in All Monsters Attack has an adventure with a Japanese child named Ichiro. Ichiro is able to visit Minilla in his dreams. Ichiro ask Minilla how to get to MonsterIsland in his dream through a radio. Minilla tells him to take a airline ticket to Monster Island. This is all done in a dream sequence. Ichiro is able to travel from Tokyo to Monster Island and vice versa after the initial ride with ease. It looks like Minilla has the ability to help others travel through the Astral plane but does not have the ability to travel through it himself.

2) Has a plausible explanation for the Tower (both his attack and his defence)
Minilla is pretty weak in comparison to Dad(Godzilla), but in comparison to others, he would be very, very strong and next to impenetrable.. He's able to hold his own against Gabara, who weighs about 23k tons and has a height of 58m. He's able to knock him to the ground and doesn't take much damage when Gabara smacks him around.

3) Has a plausible explanation for the Circus (both his act, and the reactions)
Depending on what stage of growth he's at the circus. He can look like a mini Godzilla or a pudgy version of his dad.

4) Isn't one of the impossible categories (unless it is an exception)
He is probably copyrighted. But is that an issue if you don't use his name and MITD is re-drawn like whats been done for the hello kitty umbrella.

5) Existed before strip #100 in a form accessible to Rich.
Yup...1967 Son of Godzilla/1969 All Monsters Attack(Godzilla's Revenge)

6) Size no bigger than Huge ("fits in the box")
Has the ability to change his size from Small Child(1.6M) to 18M in All Monsters Attack. As Godzilla Junior in Godzilla vs. MechaGodzilla 2 he was 1.64M.

7) Is vulnerable to mind-affecting effects (SoD)
Don't see why not. Godzilla was mind controlled in Invasion of Astro-Monster
http://godzilla.wikia.com/wiki/Invas..._Astro-Monster

Other things that support Minilla
a)Why would Xykon expect MITD to eat children? In Go! Greenman series...Minilla is sent to get children's blood for Maoh(http://godzilla.wikia.com/wiki/Maoh). Maoh is the source of all evil(could be the SNARL) as Maoh has been banished to the Underworld. Maoh needs the blood of children to be released from his prison.
b)Has the ability to talk while a juvenile, but Adult versions do not have this ability. This would explain why the big game hunters where surprised it could talk.
c)It looks like Minilla knows about travelling through the Astral plane, but hasn't been there himself.
d)Minilla doesn't really know what he can and can't do until somebody shows him how to, or under extreme circumstances (like a boulder dropped on his tail)
e)Minilla a coward and won't stand up to bullies unless his dad makes him.
f)Minilla is very juvenile and acts like a child.
g)You would expect to find Minilla on MonsterIsland with other gargantuan monsters and not roaming the jungle.
h)Has 2 brains...which might make him very intelligent.

Things that don't support Minilla
a)He looks to have immense strength but enough to cause an earthquake? If he was in his 18M size I would say yes...but as a 1.6M version? Not so sure of that.
b)He has the ability to shot Atomic Smoke Rings or under rare circumstances shot Atomic Ray. MITD has never used this ability or anything similar.
c)Has the ability to communicate with his dad over long distances. Where's his dad?
d)Would he have any knowledge of magic like Divinity? I doubt it.
e)Would he have the ability "STOP"? I doubt it.