Bitcoin has no problem when its forked. The latest example was when there has been a fork which resulted with a new currency named bitcoin cash and as you said everyone having bitcoin also had bitcoin cash at the same time which is free money. However bitcoin price is going up determined and if you have bitcoin there is nothing to worry about. Alt coins however may be acting differently on graphs. Lately ETH is going up too however some altcoins go down. A group of people cant effect price of bitcoin unless they have really serious amount of bitcoin. Please have a look at this page:

https://blockchain.info/en/stats

and see how big trade volume is. When i looked it was nearly 50k BTC which is much more than 200 million dollars. And this amount belongs to just last 24 hours of transactions. Someone or a group of people really need to have relatively noticable amount of BTC in trade among this trade volume so that they can effect prices so much or maybe trade this amount in an exchange platform with a low volume to effect prices easily. So there is possibly no one who can effect the prices directly. I think the effect you mean is much easier to do using banknote currencies in several countries but not with BTC.

I also need to mention that the reason of form which caused bitcoin cash is that bitcoin transactions are slow and they thought bitcoin cash would be a solution for that. I think bitcoin is not an ideal currency. It has advantage of being the first and most recognised so this is the reason it is so high valued today and may be it will continue for a long time. But if cryptocurrency related society has a common decision that another unit is much better for many reasons, than it can replace the popularity of bitcoin. This is possible when conditions allow.

I see BTC mostly as an investment way rather than a payment method because there are some popular altcoins which allow you to make transaction in a short time. Its better to send money using another coin (ie. LTC) so the receiver gets it much earlier than using BTC for sending money.