How to unlock all the missions?
Use a text editor to edit the "playercfg.lua" file in the "\homeworld2\bin\profiles\profile1" folder. Find the "maxmission = x" entry and change its value to the level you want unlocked (for example, setting maxmission value to 15 will unlock all missions). Then, you need to make a copy of last "persist*.lua" file (in "homeworld2\BIN\Profiles\Profile1\Campaign\ASCENSION" folder (where * is any level number that you had played). Just make a copy of it and then replace * with the level you want to play (i.e if you want to play level 6, copy "persist4.lua" file to "persist6.lua"). If you don't do this, the game finds no player ships in the next level, and it regards it as failed (Mothership doesn't exist).
Locate the file "persist*.lua" where the * is the mission number. The file is in the "Homeworld2\Bin\Profiles\Profile\CampaignASCENSION" folder. Open the "persist*.lua" file with a text editor such as Notepad. Locate "RUs =" at the bottom of the file and change the value to whatever you like.