1) It will be set to 9192 and you will see a log entry fwdd_cos_set_delay_bandwidth:queue:19 delay buffer size(0) too low, setting to default 9192
2) Total buffer size is platform and configuration dependent. I believe that for SRX650 it's 200ms and 4s with q-pic-large-buffer enabled. You can easily figure it out by yourself. Set it to some arbitrary high value and look in the syslog for COSD_LARGE_DELAY_BUFFER_INVALID in the log.
3)I was sure that using buffer-size remainder more than once will give a commit error but it works. How exacly I don't know. I can't find it documented anywhere and it's not something you could test easily. On the other hand I know that if you set it as percent the buffer size is elastic. It can expand over the limit together with transmit-rate. If you set it as temporal it's not-elastic. I believe that percent exact combination will too make in not-elastic.
Best to stick to recommended rule and set matching transmit rates and buffer sizes.
Beware that SRX will not warn you when you exceed maximum buffer size. You will find warnings in the log and interface CoS configuration will fallback to default settings.
Configuring large delay buffers
https://www.juniper.net/documentation/en_US/junos/topics/task/configuration/cos-configuring-delay-buffer-size.html
Regards, Wojtek