These winter bees have fatter abdomens (compared to summer bees), as well as a longer lifespan.

1)  这里，compared to 可以换用compared with 吗？

——可以。意思不变。

2)  as well as 是连词吗？为什么后面只有一个名词短语a longer lifespan？

—— as well as 是连词，跟 and 差不多。去掉括号里成分，一看便知：have 有两个并列的宾语 fatter abdomens as well as （以及）a longer lifespan.

3)  as well as a longer lifespan 可以放置句首吗？上句如何应该正确翻译呢？

—— 一般不放句首。compared to summer bees 可以放句首。

【翻译】与夏季蜜蜂相比，冬季蜜蜂的腹部更丰满，寿命更长。