How strict are the time validation rules?
Very.
If the next block is mined more than 2 hours after the current block, would this not stall the blockchain?
No.
It doesn't break the rule "Full nodes will not accept blocks with headers more than two hours in the future according to their clock."
Nor does it break the rule "Must be strictly greater than the median time of the previous 11 blocks."
The above should be clear I hope, but in case it isn't, lets make an example:
| Event | Time |
|---|---|
| Last block mined | 08:00 |
| Median time of last 11 blocks | 07:00 |
| Current time on my clock | 11:00 |
| Time on block arriving now | 12:59 |
| Rule | Result |
|---|---|
| (12:59 - 11:00) < 2 | OK |
| 12:59 > 07:00 | OK |
So no problem even though there's been a three hour gap instead of 10 minutes and even though the block has a timestamp that I think is nearly two hours in the future!
Note that these particular rules are not broken if there is a huge (e.g. 50 year) gap between blocks. In theory everything would continue (though it seems likely there would be other issues preventing this)