Is it possible to decrypt the entire message with one half of the key in this example?
There are two major problems with this method.

The first problem is that Susan is likely to be able to recover significant amount of data from a series of such blocks.

For example, if Susan knows $subkey_1$, then she could recover the value $subblock_1 \oplus subblock_2 \oplus subkey_2$; if a single block is encrypted with this key, she can't deduce anything from this (as she has no information about $subkey_2$; however if multiple blocks are encrypted with the same key, then she can exclusive or the values, obtaining the value $subblock_1 \oplus subblock_2 \oplus subblock_3 \oplus subblock_4$; if she has a guess about the encrypted data, this is likely to reveal more.

The second problem is that someone with the entire key is unable to uniquely decrypt.

Here's why: someone with the encrypt key could decrypt $ciphertxt_1$ to compute $subblock_1 \oplus subblock_2 \oplus subkey_2$, and thus recover $subblock_1 \oplus subblock_2$

And, they could decrypt $ciphertxt_2$ to recover $subblock_2 \oplus subblock_1$

At this point, they have the value $subblock_1 \oplus subblock_2$, howqever without any further information, they cannot recover either the values $subblock_1$ or the value $subblock_2$.  Yes, if the decryptor had some side information about either of those values, he could determine the other; however we generally don't assume that the legitimate decryptor has such side information.

Hence, this method makes things possible for the attacker, and not possible for the legimate user.