you have to be a certain tamer rank when you are that rank congratulations you can digivolve into mega
At the moment (as of December 10, 2015) the ultimate level form of Digitamamon cannot digivolve or mega digivolve to any mega-level Digimon called HolyDigitamamon the only mega-level Digimon that Digitamamon can digivolve into are Titamon which is known as a "One-Man Digimon" and has the Virus attribute and Digitamamon can also digivolve to Devitamon which is a Mega-level Digimon, has the Mutant type and has the Data attribute.
you need to have the gold tamer rank to digivolve to mega and raise them
Marcus' partner is Agumon and he can digivolve to GeoGreymon and then to RizeGreymon and then to ShineGreymon, Thomas' Digimon partner is Gaomon and he could digivolve to his Champion level form of Gaogamon and then to his ultimate form of MachGaogamon and then to his mega formof MirageGaogamon, Yoshi's partner is Lalamon and she can digivolve to Sunflowmon and then to Lilamon and Rosemon and Keenan Crier's Digimon partner is Falcomon and he can digivolve to Peckmon who could digivolve to Crowmon and then to Ravemon.
No, neither BigMamemon or CatchMamemon can digivolve to TonosamaMamemon although both BigMamemon and CatchMamemon are Ultimate level Digimon so they will be able to digivolve further to that of the Mega level but as of 2014 they do not have a known Mega level digivolution.
No, Ornismon is not currently known as Kokatorimon's Mega form however some of his Mega formswould include Ghoulmon and Machinedramon but only if he digivolves into his Ultimate form of Megadramon first additionally if he digivolves to Sinduramon then he would be able to digivolve with Valdurmon. Machinedramon, a Mega level Digimon can also digivolve to 3 other Mega-level Digimon which are Millenniummon, HiMachineDramon and Chaosdromon, Milenniummon can digiviolve further to Moon=Milenniummon and Chaosdromon can digivolve further to the next Mega level form of Apocalymon.
No, I believe its the only quick way, unless you want to use that 3% chance farm encyclopedia.
As of 2015 there's been no confirmation that Palmon's Mega form has been changed in any version of the Digimon series, Mimi's Palmon from the series is still confirmed to warp digivolve to her Mega form of Rosemon due to the Digimon Adventure video game.
it does not digivolve because it is a mega
Agumon is a Rookie-level Digimon that first appears as Tai's partner Digimon in the first Digimon season, Agumon eventually gains the ability to digivolve to his champion form of Greymon and then to his Ultimate form of MetalGreymon and then to his Mega form of WarGreymon.The next main Agumon appears in Digimon Data Squad as a main partner to Marcus, Marcus' Agumon eventually gains the ability to digivolve to his Champion form of GeoGreymon and then to his Ultimate form of RizeGreymon and then finally his Mega form of ShineGreymon.
Liollmon who is a Rookie level Digimon could possibly golden armor into a Manticore-based Digimon but because it seems to be based mostly on a lion-like or tiger-like creature and therefore one opinion believes that it would not golden armor digivolve into a Manticore Digimon however it is not known to be able to armor digivolve it can only regular digivolve and it can digiviolve to its Champion level of Liamon who does not have a further digiviolution path at this point which means no known Ultimate or Mega form.
Seraphimon is a mega level daikon. You can get the Seraphimon daikon by obtaining MagnaAngemon. MagnaAngemon will then digivolve into Seraphimon at level 66.
Both are Puppet Digimon from the Metal Empire that are supposed to resemble another species, and the Digimon they are mimicking are related themselves. The definitive answer for this question is ultimately unknown, but the logic behind it is plausible. By the way, another counterpart of Agumon, in this case his X-Antibody form, is already confirmed to be able to evolve into an Omekamon. So, maybe it could pull the same trick.