Not a direct answer just some points about this :
1 - More crops are grown specifically to feed livestock than to feed humans
so the move of people to a vegan diet will reduce the amount of crops needed
(and yes animals are fed non-edible plant parts and feces "fish flour" ect but thats on top of the crops you grow - about "grass fed" first its a tiny tiny part of the meat in genral and even grass fed are fed crops- by piles of crops put in the fields - by feeding cattle in winter time - by feeding the cows a high calorie diet some months before slaughter ect)
so if someone really cares for insects ect that's just another good reason to become vegan !
2 . A good source on the buddhas diet is : http://www.dhammawiki.com/index.php?title=Diet_of_Buddha
3 . A good video about eating meat is "animals and the buddha" :
https://www.youtube.com/watch?v=S0MWAAykFuc&t=254s
4 . This is hard to explain in non-buddhist sites but here its simple : INTENTION to kill is key - a lot of the deaths in crops are not intended to be dont (some are but not all) unlike the deaths in the meat industry which are intended - so when you buy meat you pay for someone to kill you pay someone to cause some very bad karma
So INTENTIONAL KILLING vs UNINTENDED KILLINGS ..... and knowing for sure that when you buy meat its going to cause intentional killing
and yes its not like killing the animal yourself ... but still there is a difference - and this is another thing people should take into account
5 . i follow theravada buddhism so i talk from this point but i know there are vegetarian sects in mahayana buddhism - but maybe im wrong didnt research this - but my view in theravada is you can eat meat - but it is conducive to avoid eating meat completely and being vegan
6 . monks and lay people are very different and theres a different between a monk accepting meat and a lay person buying meat at the supermarket