iMessage time stamps messed up

Ok. I get the picture now. The Messages app and iMessage service are doing exactly what they're geared to do in that situation, date/time stamping included. The problem is that you're either getting a false negative submission acknowledgement or no submission acknowledgement at all from the carrier on the original SMS. If it's no submission acknowledgement at all, that could be due to the carrier using a lower level acknowledgement system not supported by iPhone and thus probably not widely supported by smartphones. This could indicate a non-iPhone certified carrier service. Dirty deeds done dirt cheap.

I reiterate that when I temporarily disable cellular data and Wi-Fi (regardless of whether Wi-Fi is available) and send a message to any contact including iMessage contacts, the message originates and sends as SMS with positive submission acknowledgement. There is no suspended iMessage to follow up on. "Send to email address" might be a new option with iOS 12. Lack of "Send as Text Message" indicates that either that the message has sent or else SMS is unavailable.

I'm confident that iOS has no affect on the core cellular voice or SMS. I recommend you read your carrier service contract and visit your carrier's support site and find out exactly to what extent your specific service subscription supports SMS on the iPhone. No way around the limited data and SMS acknowledgement problems without upgrading the cellular service plan even if that means switching carriers. If you do upgrade your service with your existing carrier, compel them to burn you a new SIM to make sure your SMSC steering is current.

I guess iMessage is geared to do its thing if it can, when it can unless you signal it not to and the way to do that is to temporarily disable it by toggling it off in Messages settings on the iPhone whenever viable Wi-Fi is unavailable. Note that toggling it off doesn't sign you out so you can quickly re-enable it by toggling it back on any time. Just curious, though. When the message is ultimately sent as iMessage, does the original SMS remain or does it disappear or get replaced by the newer iMessage? Also, I'm guessing you're forcing the original message to fall back to SMS by temporarily disabling cellular data so let me know if that's incorrect.

When you do this, also temporarily disable Wi-Fi even if no Wi-Fi is available. Then the message should originate and go as SMS as soon as you hit the send arrow. That should give you effect you're looking for. If not, then also try updating iOS to the latest version which came out a week or two ago and also check for carrier settings update by going into Settings > General > About. Presumably, if a carrier settings update is available, you should get a pop-up saying so.

I did some testing. I disabled cellular data as well as Wi-Fi on the iPhone and sent a message to one of my regular iMessage correspondents. It originated and immediately sent and was delivered as SMS with no latent iMessage retroactivity so far. Then I re-enabled cellular data as well as Wi-Fi and disabled iMessage and sent a message. Same result but with the catch that disabling iMessage disables Text Message Forwarding so this one didn't get forwarded to Messages on my Mac. I also made sure iMessage was working normally with everything enabled before and after testing.

Although the latent, retroactive iMessage makes sense, at least conditionally, the original SMS, if actually acknowledged as sent, absolutely should not disappear or be replaced by a later iMessage. Yet, if it's not actually acknowledged as sent, it should be marked as Not Delivered ! sooner or later. So this sounds like a carrier SMS issue e.g. Delivery Acknowledgement (on top of the requisite Submission Ack) is enabled in carrier settings but not on carrier SMSC. Long story and a manifold measure of imagination as to how that could lead to the effect you're experiencing.

So you may be able to exactly get the effect you're looking for as stated above. On the other hand, there could be a carrier SMS issue at play that may cause you to have to live with the effect you've been experiencing.

That makes it sound like SMS is unavailable. With Wi-Fi and cellular data both disabled, you need to see 3G or something like that where the Wi-Fi icon would normally be. Maybe you're out of range or in Airplane mode. Otherwise it's a carrier issue. iMessage can override it when Internet is available but it can't defeat it when Internet is unavailable. Or maybe it's the way you're addressing the message. It needs to be to a cell phone number for SMS to work. Try tapping on the create new message icon and manually entering their cell phone number.

Ok so that's an indication that SMS is available and working. So, although iMessage shouldn't defeat SMS to iMessage contacts when Internet is unavailable, apparently it can, probably due to some kind of bug. Particularly if the problem persists even when you start a new message and manually key in the phone number.

Update: Lack of "Send as Text Message" indicates that either that the message has sent or else SMS is unavailable which may be due to Send as SMS being disabled in Messages settings.