You're not alone with the Motorball thing; I have this curse where any boss that appears at the end of a lengthy segment (Jenova LIFE being the other one) hits a difficulty spike that I didn't want. What I should do is review how that segment works, but I'm not sure what would be an appropriate replacement. Motorball has to be fought at the end of the motorbike game and it's kind of the climax to the Midgar Escape, so...
There is a shop available when Aeris reaches the first floor, up the staircase on the second floor (as well as the new shop on the floor where the Gunners are fought) but as random encounters are disabled at that point the player might not be able to stock up on Potions/Phens if they have no money left (seems the Poverty Mod is living up to it's unofficial nickname :l). I'm wondering what I can do about the Ether thing though; I need to get players to make use of physical hits a little more to ease the strain on MP, probably.
A good alternative to magic is [Element]+Elemental materia in the weapon and a stack of cheap Grenades; both together can hit for a nice bit of damage when Limits pile onto it. Yuffie is joining soon with the Time Materia and a Slow-Haste advantage over a boss can greatly boost your damage output, especially if you grab some Carbon Bangles/Shinra Betas to up your Dex stat. That can be good for phys-set-ups in the early game. In Junon, I'd recommend checking out the shop there; he sells Echo Drops now that'll be useful for the boss (otherwise, Capparwires in the nearby forests/beaches carry them).
Don't worry about fighting a lot; there's 'hard caps' in the levels that are difficult to get through on random encounters alone. Bosses cut into these caps quite well and so far it seems to have performed the miracle of making grindy and not-so-grindy players stick to a similar level. It should hopefully not be necessary to grind at all until you reach the Crater but try not to rush either.
Thank you for your advice. I just got done defeating Jenova-VECTOR after losing an ungodly number of times. I ended up using all 30 of my Potions and even then it was a really close thing. With her down to less than 1K HP she used this move called...Electrosurge maybe? Whatever it is, she ALWAYS used it twice in a row. Cloud dead, Yuffie dead, Aerith barely alive, I somehow managed to revive Cloud and Yuffie and scrape out a win. I have no Ethers, I used up this Ghost Hand I stole earlier as well as a Turbo Ether I stole from..Rude I think. Or Elena. One of them.
In short, I'm right screwed at the moment in terms of MP restoring items. I also broke myself restocking on Potions and Phoenix Downs so I'm in a bad way. I suppose I'll need to start buying Hi potions soon but right now 300 HP is definitely enough to keep us alive.
I did take your advice though. Once i realized she was weak to Earth, I had Cloud as my main damage dealer with Earth-Elemental doing 200 damage a swing.
Also question - does every boss have a couple items to steal? I notice this when I die several times and repeat the boss. Like, Motorball had a Platinum Bangle and some other stuff. Jenova VECTOR I think had a Fire Ring but my successful attempt ended up with a Safety Bit.
One more question - would you recommend Sadness as a way to deal with damage? Or if I'm depending too much on that it means I'm doing something wrong? I've been avoiding using it for precisely that reason.
EDIT:
Looking at a video of another person fighting Jenova VECTOR, although they had a completely different party than I did (Tifa and Barret vs. my Yuffie and Aerith) they also seemed to get more hits in, even without Haste. Perhaps I need to prioritize Dex? I figured Defense was the most important thing so I didn't bother with getting any Shinra Betas... I'm dumb. I still have the thing Yuffie joined up with though but she's only 1/3. The other two will still be too slow it seems.
I also bought another Restore Materia but maybe I should have used my money on some nw attack materia, like another Ice or Fire or whatever. I'm just a very defensive, cautious player and I should be more offensive.