Although according to their FAQ you could get your transaction mined by other pools because they have a cooperation with other multiple pools but I doubt it will be for free services, it will certainly be for those paid services because they could be paying commission to those pools too. In order to pay miners it will be logical to just use the fee rate estimate by mempool.space as you said or wait till they get less congested a bit if one is not in a hurry to receive the transaction
Yeah, you are right, if you use ViaBTC paid service, your tx can be included in a block mined by one of their cooperative pools.
After receiving your acceleration fee, ViaBTC will notify all cooperative mining pools.
But rather than using the paid service for tx acceleration, it is better to have checked the mempool before broadcasting in order to pay an appropriate fee, or you bumb the tx with rbf if it is stucked.
Well, if the mempool is congested like it is now, will it take longer to mine the block too ?
Yes, I know that the transaction fee is increased but due to this, the block mining time is also increasing ?
BTC blocks are mined ~ 10 minutes on average. The network is congested because fee rate is high and people who attached low tx fees will have to wait as their tx will be unconfirmed, but if you use the fee rate for high priority, your tx will likely be mined in the next block.