Is ablution valid when the foot is touching water from below, bearing in mind that the intention comes in the sequence of the (ablution) acts?

Your ablution (wudu') is valid even if your foot touches the water from below, as this does not violate the order of the ablution parts. You did not intend to wash your foot from below before the parts that precede it. What is important is to completely cover the foot with water and perform ablution thoroughly. It is worth noting that washing the ablution parts in order is a Sunnah according to some scholars and not an obligation.