We do not have accumulators on our cars. That is used for a CCOT system. TXV systems such as ours call the reservoir a receiver. But most shops are used to American iron, which pretty much all use CCOT systems.

You NEED a new dryer to convert to 134a. The 134a-compatible dryer works just fine for R12, but the reverse is most definitely not true. Your old dryer may be restricted as a result, or even worse, it could have spewed its guts into the evaporator.

You can't expect "the system" to accept refrigerant with the engine running if the AC compressor is stopped, unless you take special steps. You CAN introduce liquid refrigerant into the static system (car turned off), and that is what I prefer to do. But expecting a non-rotating compressor to pull refrigerant into the system is expecting magic.