My brother _____had been on a visit to Paris ,returned last

My brother _____had been on a visit to Paris ,returned last week .
此处为什么只能用who ,不能用that?在限制性定语从句中,关系代词在从句中充当主语,且先行词为人,不是that/who都可以用吗?而且此处be on a visit 不是一个短语吗,又不存在从句有与that 搭配的介词?为什么不能用that呢?
The TV tower _____ is as tall as 200 meters adds to the beauty of our city .
为什么这里填where,不是which/that?
Can you give such lectures ____will interest us
这里为什么不能填that ,而只填which?