1. Can the stay provided by HC (30 day surrender period) be extended?If yes on what grounds?- This can be extended by HC but the real issue is that once your period of 30 days is lapsed then the other side may try to get you arrested so the period in between when the extension of time is applied and it is granted by HC would be a caution period within which you need to be caucious. The extension is allowed normally any genuine grounds may be cited your HC lawyer would be able to suggest the valid grounds. This is a simple application that need to be made for extension of time. The same case your writ in All HC that would be having a disposed status would be listed and judge would grant further extension but that would be last and would not grant any further extension after that. So even if you miss on the first 30 day timeline to surrender then too you would get a second extension.
2. Should husband first file SLP in SC or TP in HC in 498a matter? Anything that he wants to as these are just time gain tactics or you can do both simultaneously as SLP would be for challenging the HC order for not quashing the FIR due to mediation failure but the main thing you would be looking for would be to get the stay of arrest extended from SC and also getting the lower court proceedings stayed.
I think file transfer petition later when your SLP gets decided the benefit being that SC may take 2-3 years during which your stay will be valid and lower court proceedings stayed and once you loose here you can file a TP claiming jurisdiction issues in HC - which as a practice would refer to mediation and stay of arrest would be again granted. This would agin give you 1-1.5 years delay and thereby relief from court proceedings.
Even if your case being a complaint case still chargesheet will be filed by police sometime after investigation so once filed you can again file a case in HC for quashing of chargesheet u/s 482- this will again get you stay of arrest.
If you manage to get this case stayed like this for 3-4 years you will be relieved from travelling to lower court and seeking bail plus also the other side would be pressurised that may result in settlement.