You should clarify your role with the former client as it may affect the advice you provide. Do they want you to look at everything, soup to nuts? Review the plan "termination", the "merger" or transfer, the receivable issue and anything else related? To give proper advice you should tell them this is what you need to do and what it will cost.
OTOH, if they are just asking you if they can deposit the receivable to the surviving plan without reviewing anything else, you should caveat your response that it is not based on a complete review of the situation and might not be correct. That said, can they do it? Probably, as Bird notes, under a merger it should be OK, but even if it was terminated and transferred, under the principles of self correction it's probably good enough (but IRS could take a different view under exam). There may be some make-up earnings due as well, but unless they want to engage you to figure out if so and how much, i'd just point this out. And presumably Company A adopted the surviving plan? As Bill says there's a good chance something was done wrong, you can be helpful, but just be clear on the limitations of your assistance.