Hi there and welcome! Im afraid nobody cant help you with the wingman. AI is terribly bugged and it would be up to ED to fix that, nothing you can do afaik. Also, afaik, AI wingmen cant rearm or refuel...
But then, I dont fly singleplayer really, so maybe others have some advice for you.
I can however probably help you with your second problem, you SPI 'jumping'. This annoyed the hell out of me as well when I was new, but there are two simple workarounds that I use.
1. the probably correct one (Im not a real pilot..) is to make a markpoint on your target. I would try to get into the habbit of this really. Once you find a target on the TGP, instantly go TMS right short and you have a markpoint. Now for your attack, make your new markpoint your steerpoint, this also gives you the advantage of using the HSI now to set up for a correct final attack heading. If your SPI 'jumps' at any time, no problem, simply slew your TGP back to current steerpoint using CHT aft long.
With proper binding of switches, you can do the above procedure without using the mouse, so once you get into the habbit of that, you will be super effective and never lose a target again.
2. Another more easy way for a beginnger is to simply use the INR mode in the TGP (TMS down) instead of Point or Area lock. With INR, the TGP will always get back to the same location and will not be 'jumping' due to paratial terrain masking (as in when you evade a sam). Disadvantage of course it then it wont track a moving target, but its still better than resetting the SPI completely
Hope that helped
edit: sniped by Cik :D I agree: markpoint markpoint markpoint!! :D