On the bright side, items you buy in mp stay bought. If you're really that torn up about it. Bash an easy ai in custom a few times with these settings:
gamemode conquest
gamespeed fast
towers weakest
minions strongest
starting citadel level highest
starting gold highest
spawn rate highest
Those are the important ones, you just have to buy all the way up to giants in your citadel and they'll end the game for you in ~ 2 minutes gametime(less in real time i think). It takes a little while for changes to catch up, but you get pretty much at least 130 favor per round.
This is why favor points won't be granted in customs later on btw 
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