1- In general, film is fairly forgiving. More so of overexposure than underexposure. With the underexposed rolls adding extra development (pushing) is prudent and reliable for making negatives more usable but, it won't create detail in the shadows that doesn't already exist. Also time is working against you. The fresher the latent image the better the detail will be. The developer you have, used today per the manufacturer's directions for a push, may do a better job than any specialized developer that takes you a few weeks to get. There is a fairly fresh thread about this effect, search "latent image".
2- In general, film follows the rules of physics very nicely. Problem is we don't always have control of or manage all the variables/rules well. You are far from being unique here, we have all done this. There are so many variables that I find that with a problem like this that it is best for me to only deal with what I know I did, using stuff I already know how to use.
What I'm saying here is that experimenting with stand development and new developers, different agitation, and other things may be interesting but it is no gaurentee. Adox, Ilford, Kodak, Rollei, et al know their products well. When in doubt following their recommendations, tempered with your own experience, is quite reliable.