All they need to do is to have 2 separate player profiles...1 for Portal and the other for MP, done.
Then allowing full XP for any portal match (including farms) is fine. You end up playing in Portal just for playing the match and unlocking weapons. attachments and skins to use (in portal only).
So you go in realizing that any sort of PvP inside portal is like the wild west and doesnt matter and you dont play Portal for stats...only for fun. So let them remove stats from portal matches, but leave the full XP and weapon/attachments progression in place.
...OR, they could also just remove all xp and progression from all portal matches and have all game content (weapons/attachments) already unlocked (as they do in cod private/custom matches with bots). This method is ok too.
Just give us back Portal with full field of bots, in all Verified game modes and no requirement for more than 1 player to start the match.