为什么呢______himself at the desk,he began to type his resumesit

楼上的思路正确.
在汉代英汉词典中,sit可以作及物动词,例句Just sit me up a little.以此分析,前置的从句是he sit himself...,省略的是he,与主句主语一致,用现分形式.
seat也是vt,在简明英汉词典中解释比较详细.同样道理,本句中也可以用seating,但是不能用过去分词.
Seated at the table by his mother,he began to type his resume.
这样可以.
如果从句主语是himself而主句主语是he,那得用独立主格结构.比如 With himself seated at the table,he began to type his resume.这样从语法上来讲符合规则,但是实际上没见过这么麻烦的.
我看你的问题似乎是认为he与himself属同一主语,实际上不是的.通常he主动而himeself被动.
Q：27683672

我觉得如果没有himself就应该用seated,现在有宾语嘛。
简单说，当你觉得用seated很怪的时候就用seated,正常的时候就用sitting.
你还是问高人吧，我也不太懂。