grammar - If the lava comes down as far as this, we will evacuate these houses

Someone wrote to me as follows.

(a) If the lava comes down as far as this, we will evacuate these houses.

(b) If the lava will come down as far as this, we will evacuate these houses.

Syntactically, both (a) and (b) are correct.

But semantically, (a) is not understandable because if the lava ( from the volcano ) comes down as far as THIS, it will be too late to evacuate THESE houses.
So if the lava WILL ( = is likely to ) come down as far as this, we must evacuate these houses immediately.

I think interpreting (a) as the person does is unnatural.
I would like to know native English speakers' opinions.

A similar(but different) question was asked here(https://ell.stackexchange.com/questions/60972/if-the-lava-will-come-down-as-far-as-this-we-will-evacuate-these-houses).

EDIT(21 July 2015)

Perhaps explaining the backgound of my question might help.

We were discussing which sentence is correct, (a) or (b).
I think (a) is correct and (b) is incorrect,
because there is the following grammar rule. http://www.englishcafe.jp/englishcollege/etense2/e1-2-4.html "In the time and the conditional clauses, both of the present tense and the past tense are used, but "will" and "shall" is usually not used even if it means the simple future. It is inevitable that the present tense is used instead."

However some people say that there are exceptions that we use "will" in if-clauses.

Practical English Usage §260 If … will

We normally use a present tense with if (and most other conjunctions) to refer to the future.
I’ll phone you if I have time. (NOT … if I will have time.)
But in certain situations we use if … will.

1 results
We use will with if to talk about what will happen because of possible future actions – to mean ‘if this will be the later result’. Compare:
–
I’ll give you ￡100 if I win the lottery. (Winning the lottery is a condition – it must happen first.)
I’ll give you ￡100 if it’ll help you to go on holiday. (The holiday is a result – it follows the gift of money.)

–
We’ll go home now if you get the car. (condition)
We’ll go home now if it will make you feel better. (result)

2 ‘If it is true now that …’
We use will with if when we are saying ‘if it is true now that …’ or ‘if we know now that …’.
If Ann won’t be here on Thursday, we’d better cancel the meeting.
If prices will really come down in a few months, I’m not going to buy one now.

3 indirect questions: I don’t know if …

We can use will after if in indirect questions.
I don’t know if I’ll be ready in time. (NOT … if I’m ready in time.)

4 polite requests
We can use if + will in polite requests. In this case, will is not a future auxiliary; it means ‘are willing to’.
If you will come this way, I’ll show you your room.
If your mother will fill in this form, I’ll prepare her ticket.
Would can be used to make a request even more polite.
If you would come this way …

5 insistence
Stressed will can be used after if to suggest insistence.
If you WILL eat so much, it’s not surprising you feel ill.

EDIT 2(21 July 2015)
Some people say it all depends on context.
They say (a) may be incorrect and (b) may be correct depending on context. But I cannot imagine such a case.