Your electrum client will be able to confirm that that any transaction data returned by an electrum server will be accurate, however you will need to place trust in the electrum server in a sense that they will fulfill your request for data, and if they do not, then your customers could be negatively impacted. Within the electrum servers that public limits, the highest limits among electrum servers generally tops out at 10,000, and I would recommend to create a new electrum wallet when you approach this number of received transactions.
If you run your own powerful electrum server, you can take steps to prevent the general public from utilizing your server's resources, and it may be able to handle requests of this magnitude.
I would also point out that attempting to run *any* wallet software that is monitoring 100,000 addresses will require a very large amount of resources.