I've not come across such issues to date.
1) What will the contractor do when/if the plaster finish cracks as a result of using a lightweight block? (I understand that lightweight blocks wick the moisture out of the plaster quicker, which can then lead to cracking.)
2) Can't the 5-6 courses limitation be managed? (How many linear meters of block work are there to lay in one course, and how long would it take to do a complete circuit?)