E.g. is it within actual powers of the government under the circumstances
Absolutely not. The Civil Contingencies Act 2004 is extremely clear in defining the kind of circumstances in which its provisions can be invoked.
19. Meaning of “emergency”
(1) In this Part “emergency” means—
(a) an event or situation which threatens serious damage to human welfare in the United Kingdom or in a Part or region,
(b) an event or situation which threatens serious damage to the environment of the United Kingdom or of a Part or region, or
(c) war, or terrorism, which threatens serious damage to the security of the United Kingdom.
(2) For the purposes of subsection (1)(a) an event or situation threatens damage to human welfare only if it involves, causes or may cause—
(a) loss of human life,
(b) human illness or injury,
(d) damage to property,
(e) disruption of a supply of money, food, water, energy or fuel,
(f) disruption of a system of communication,
(g) disruption of facilities for transport, or
(h) disruption of services relating to health.
(3) For the purposes of subsection (1)(b) an event or situation threatens damage to the environment only if it involves, causes or may cause—
(a) contamination of land, water or air with biological, chemical or radio-active matter, or
(b) disruption or destruction of plant life or animal life.
It is beyond clear that remaining in the EU for a period of 3 months meets none of the conditions the act supplies, without with its provisions cannot be invoked.
Simply put, anyone saying this is a possibility is either clueless or fear-mongering.
Regarding your additional question:
Can the Supreme Court meet in some kind of emergency fashion, assuming that a first instance court would side with Johnson somehow (e.g. they would give the government the benefit of the doubt that widespread rioting would occur if Brexit is not done, something that UK government sources have claimed at one point.)
Note that I consider this a very unsafe assumption; the law would really be very very clearly not on the government's side in a case like this, and the CCA mandates regulations made under that Act are Statutory Instruments, which means they can be quashed by the lower courts. Tl;Dr there's not really a tight time limit because the lower court would almost certainly* quash the SI that revokes the Benn Act anyway.
The Supreme Court's website goes over the process of arranging court hearings here. The enigmatic reference to emergency hearings is:
Requests for expedition
6.2.4 Any request for an expedited hearing should be made to the Registrar. Wherever possible the views of all parties should be obtained before a request is made.
This presumably implies that hearings can be expedited where the court feels it necessary. That's up to them of course, but they presumably would do so in this particular case. Jolyon Maugham, who is one of the lawyers for Joanna Cherry in the prorogation case) has been vocal on his twitter feed on a number of occasions that the SC can move very fast when necessary, though I'm struggling to find the reference.
*But I mean seriously, who knows?