Is your modified 1405 using the stock driver?
Your 1405 situation sounds like a short-circuit to me - a short-circuit which involves two batteries in series, definitely not fun at all. My advice to you is make sure your batteries are still healthy before you use them again, check if they still can be charged without overheating, and see if the batteries still can hold the charge for more then 24 hours without significant drop of resting voltage.
I stacked one R100 resistor on the stock driver and it has killed two of my emitters in a split second due to excessive output voltage - something is not right after the resistor was added. Now I label the stock driver as scrapped part so that I won’t forget and reuse it again. Everyone should be reminded that the stock driver of 1405 is not good for high current application. ohaya and wight have done some test and analysis this thread and it is found not suitable for current boosting. I know some members may have good result with the stock driver anyway but I think there is still certain risk associated with it. And I don’t think LD-4B is good for current boosting either.
To put it simple, we don’t have a reliable and high-current capable 17mm buck driver yet.
26650 has the same voltage as the 18650 (neglect the different chemistry), so yeah you can use two 26650’s with the LD-4B driver without any issue, given that your LD-4B is functioning properly of course. By adding another extension does that mean you will use three batteries? So the input voltage will be 12.6V? If that is the case I believe this is still falling within the specification of the LD-4B thus acceptable, but I have not tried and confirmed this yet so just take my words as a grain of salt.
Messing with lithium batteries especially in series could be a dangerous thing, please be careful with everything you do.