Portis has a contract of $14.5M so he would fit in the full MLE which is $15.1M. In theory, any team with the full MLE available (and there are several) could trade for him without sending anything back. You may be right that Portis could be off loaded fairly easily. I am not sure I agree that they could get a first round pick back, but they probably could clear his salary if they wanted to and get a young play back.
Kuzma would be harder to off load ($20.5M). He does not fit the MLE so someone would need a sizable TPE (which BOS has). It is true that Kuzma is an expiring contract so MIL could just suck it up for one season, but it does not seem unreasonable that they would look to move his contract as part of a Giannis deal. They don't really need to but they may be looking to create some cap space to allow some flexibility.
Kuzma would fit the BOS TPE so maybe that is what MIL is pushing for BOS to take on Kuzma. BOS can't take on Kuzma without sending something back. They would need to use the TPE to take back Kuzma and that would hard cap them at the first apron. If they took Kuzma back for Hauser, it would add $11M to BOS cap. This is workable, but limits their flexibility if they want to stay under the tax. It also works with Pritchard going back.