(1) Yes, munfarid (the person offering salah alone) shall combine both of them.
(2) It is not specific with fardh salah only; it also includes sunnah and nafl salah.
(3) It is sunnah to say ربنا ولك الحمد after سمع الله لمن حمده. If someone forgets to say ربنا لك الحمد then there is no need to repeat the salah.
Allah knows Best!
Darul Uloom Deoband