Yes you can use AC through and no charging.. on selector switch.
It is supposed to run as a UPS....exactly as you described...... but I have not used it like this.... others may have.... but it claims to run as a ups, and either charge the batts at the same time, or just throughput and not charge.
I know it does it, but do not know how reliable it is on back to battery when shore turns off. ..... ask the seller... save response... so you can claim warranty if necessary. They are good on their warranty claims apparently.
Wish I could be more specific, but i just don't do this regularly.... and when I do, I sometimes I let the smoke out of it.... but usually doing a few other evil things at the same time making it difficult to know why it happened.
Usually I have been using the charging at the same time ( internal), and it may be that that causes the change over trouble. It is flawless on battery to ac throughput, it syncs very quickly, and changes over fine... at that point I have had it charging the batts hard, then drop the shore power... and poof...as it goes back to inverter only mode, things go bad for me.... I am sure it is the back mmf in the torroid... but have not bothered to research further, as I don't do that anymore.
As you will not be using the transformer in pass through, no charging, it may be totally different. I don't know.... wish I did.
.............oztules