reliably sync preset msg, contacts from website to 66i?

For the 66i, it's the Explore app. GCM does not sync anything with the "explore universe". 

yes, it wants some kind of account, but it looks like this can be anything.

I did now 1h drive with car. 2 66i  on board. The associated phones were switched off in my bag.

On arrival, I thought I can simply swithc the phones on and they would still be paired with the 66i.

But no, both of them were not paired any more, I had start all from scrp.

Is this normal? Or is there some trick to keep phone and 66i paired even after both were switched off?

Cannot speak to Android. With iOS, the phone itself will remain paired with all BT devices regardless of the power status of the phone or the device. Similarly, any apps using the pairing (for example, Explore) also remain "connected" to the device most recently used. 

I cannot imagine that this behavior would be any different with Android. But I have no experience with it.

I'm not sure what you mean by "... it wants some kind of account but it looks like this can be anything...".  First off, what is "it"? If "it" is Explore, then this is false. In order to sync with the right thing, the Explore account must be the one with which the device is associated. If "it" is GCM, you really should match the Explore account. Like the Garmin web properties, I think the mobile apps try to "share" login information. If you must run GCM, it would be safer (that is, it would eliminate another variable) if you used the same account for both GCM and Explore.

for the BT:

yes, the phone itself is still paired with 66i next day (setting of the phone BT)

However exlore can not find the device and so does GCM.

Those are standard Samsung Android 9 phones.

2x66i and 2 phones, both same. After I switch all off, I have to:

delete the 66i from the BT list in phone settings on android

delete the phone (unpair) from 66i

delete 66i from GCM and explore

start again and pair all step by step

as far as GCM, what does it sync when it says it is syncing?

explore related data: preset messages, are they also explore data?

If so how do I get them onto the 66i? I have edited them on the website so they work to my needs.

But I am not able to sync them to the 66i. I tried to set up GCM and explore and BT from scrab. Entered all details again. Also phone and 66i have connection to wlan. BT connection seems to be here, the wheel in explore spins for abt 6 sec each time I try to activate it, but nothing. Preset msg configuration will not bechanged.

Where is the secret?

One more Q: what means when the sync spinning wheel in explore is red and what if it is grey?

Most likely, you have to actually change the text of a preset message to get it to sync for the first time.  It actually does not matter if the text on the device appears to be correct or not. The distribution list (those who will receive the message) will never show up on the device.

When you send a preset message, a "present message number" is sent to the server. The server finds that message number, picks up both the text and the address list from the server data, and ships it out. Bottom line is that nothing on the device, other than the preset message number, matters. This is why (I think) it's sometimes difficult to get the message text to sync for the first time.

I don't remember the color coding scheme in Explore for sure. IIRC, "red and not spinning" means that you can tap it to start a sync. "Red and spinning" means that a sync is in progress. "Grey means that no sync is possible right now" - reasons vary.