Say you have a neut sailor and his support stats are 10 rep 10 res and 10 eng,then you class him to a support sailor which boosts all his support stats by about 8,so now hes 18-18-18.Then you class him to repairer which reduces the res and eng stat and boosts his repair stats and maybe ends up something like rep 25 res 11 eng 11.After each level your sailor is growing at a higher rate of repair then any other stat so becomes better at what youve chosen him to be than anything else.
So q1 is c and q2 isb.

The engineer is the only support sailor that improves engines ( OH speed and time).

As for the second part, it is not a realistic question, a repairer has such a high repair ability growth rate that a restorer or engineer will never keep up with his repair. Nor will an engineer or repairer ever keep up with a restorer for restore, but that doesn't matter since NFNA has a cap on restore, making the restorer a useless sailor.

A repairer will enver ever work towards the engine, only engineers can do that.

And yes if you manage to get 2 same stat support sailors to 120 and class 1 as repair and 1 as engineer they should provide the same repair.
Why you want to do that to a repairer is beyond me tbh. If you want a light repairer, you could also have slashed a classed-on-time repairer for some of his crew and still come out at 120 with a better repair rate.

"abilities of repair (...) are the same (...)
1.which one has the best repair rate?"
These are contradictory since same abilities of repair = same repair rate. I assume you weren't asking that.

The empirical formula is (true ability) = (4×veterans+experts) x (displayed ability) x (% of total sailors).
Assuming equal vets, equal experts, and full rookies on all 3 sailors, the one with the highest displayed repair ability will have the highest repair rate (probably the repairer depending on how long hes had the job).

If what you described, all 3 somehow having the same repair ability (e.g. a lvl 80 eng, lvl 60 support, and lvl 30 repair all with the same displayed ability), and somehow assuming they have the same number of VE, they will have the same repair rate regardless of class. This is not realistic because of their level differences, they would have different VE.

Also, 120 Support sailors make the ideal US AA gunners.