Ubuntu Alternate/Server installer cannot be copied simply to a USB flash device then install it. The installer failes when it tries to detect the "CD-ROM".
Without external tools more installer (except desktop CD what contains Casper) cannot be put on a same USB flash device.
MultiSystem is a third party tool. You can use it easily, but you don't know what were modified in the installer. Also it puts a lot of things to the USB drive what are unnecessary in most cases.
Putting a Desktop ISO to the /boot and boot it can be done without any modification of the Desktop ISO. The problem is with the Alternate and Server installers.
I suggested an official solution. With it you can simply unpack the Alternate/Server ISO into a custom directory on the USB drive and if you set the proper parameters in grub2, then you can boot it.