Settling offline transactions
I'd say there are number of issues at play here. If the system is "offline" then the only update you'll have to balances is whatever was last stored. The assumption here is both sources are trusted, and the system in the middle is trusted. Might as well make it a centralized asynchronous system. You'd have far more control and less reliance on 3rd party tools at that point.

The system proposed would be "nice" but a solution in the meantime would be to develop an internal ledger. You know party A, and party B. You know their balances, and you trust their transactions. This system assumes you know their keys as well, or can obtain them as needed. 

Based on the most recent balance store your transactions in a normal DB. Running balance, again based off most recent information. Whenever the node is able to connect, update the current balances (trust but verify), check any previous transactions you may have submitted for failures (lack of gas, improper transaction, lack of funds etc), then process the transactions. 

If there are A -> B and B -> A transactions then you could resolve transaction balances internally (assuming that is legal for what's intended). Otherwise you can put in all kinds of logic to resolve each transaction. To summarize: 


Use an internal system for tracking what's what. What's been paid, what needs to be paid.
When node connects, get as much info as you can. Current balance, confirmed transactions from previous submissions, transactions that are still waiting to be confirmed. Identify transactions that outright failed for whatever reason (this is assuming a short 'up time' for connection with the node. It wouldn't stay connected long enough to see transactions hit the first block), still sitting in mempool (not enough gas to be picked up and go stale), etc. 
Process based off the information above. Transactions will ultimately be resolved or unresolved. A FIFO can be used to resolve the oldest transactions, or if the ideal situation is to resolve the greatest number of transactions first regardless of age then a large transaction may get backlogged while many other transactions get taken care of.


This is all a matter of striking a balance of "trust" vs "timing". If i own 2 wallets, and i'm the only one with the keys, i could run every transaction in a back end system tracking what "it should be" and resolving it whenever i want. 

The provided suggestion is a balance between available technology(and circumstances), security, and accuracy.