Is it safe to charge tablet from phone via OTG cable if tablet's adapter is 5.2V? Is it safe to charge tablet from phone via OTG cable if tablet's adapter is 5.2V (a bit above)?
Yes, as long as it is supported by the tablet and you have compatible cables available. 

USB Standards specify that charging voltage should be 5.00V ± 0.25V, i.e. 4.75V~5.25V.

Upper limit is 5.25V which is higher than tablet adapter voltage ; add to this the resistance offered by the cable and the input voltage will be well within safety limits. 

How efficient it would be is upto you to verify as Robert pointed out in comments