> 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 (barring some other issue - which seems likely)