1) This depends on whether your are in THUMB or ARM mode, as these have different opcode sets. This stackoverflow question might be relevant.
2a) I don't know any online assemblers for ARM, but for single instructions, i find this table handy.
2b) If you want to assemble larger chunks of code, probably the easiest method to get a working ARM assembler is installing the Android NDK, which has toolchains for various processors including x86, arm, and mips. As i'm using Ubuntu, my arm assembler is $ndk-root/toolchains/arm-linux-androideabi-4.6/prebuilt/linux-x86_64/bin/arm-linux-androideabi-as; on windows, i'd assume a .exe file should be in a similar location. My answer to this question shows how to use arm-as and arm-objdump to get the hex bytes quickly.