Can I increase CPU power of my rMBP 13 by plugging to another computer (target disk mode or other)?
To answer your questions quickly:


Yes
No
Very likely
Probably
Not really
Maybe


Explanation:
Note that this is assuming you are booting the laptops SSD drive from the iMac. Basically this operation allows you to use the internal SSD as an external hard drive. If you boot into this drive from another computer, you are now using the resources that the computer you are working on offers. (If you go to about this Mac, you'll see the specs of the iMac you are now using.) This is definitely possible to do, and because thunderbolt speeds are at least 10Gbps (faster than the read/write speeds of the PCIe SSD), the thunderbolt cable will not be a bottleneck. It is very likely safe to do, although you should ALWAYS have a current backup in case something goes sideways. Most software should be ok with the switch, as the SSD's ID is still the same, but it is something you should test out by trying. Again, have a backup in case something goes wrong. Your only other option would be to get a new computer. The performance gain by getting the higher end 15" MBP might work, but iMacs will likely have more power than any laptop because there is more cooling space and less power worries, so Apple can put faster processors in.

TL;DR: if you have an iMac available, that is a great idea! If you'd have to buy an iMac to make this happen, you might want to explore other options.

Please let me know if something was unclear or you have any questions/comments/clarifications about my answer.