They stipulated that he should lead a communal du‘aa’ and not place one hand over the other; should he lead them in prayer or leave them?

Firstly: 

We appreciate your keenness to do good, follow the Sunnah and
avoid that which is contrary to it; we ask Allah, may He be exalted, to
enable you to do that which pleases Him.

Secondly: 

We should differentiate between that on which the scholars
are unanimously agreed that it is a reprehensible innovation (bid‘ah) and
that concerning which the scholars have differed, such as about placing one
hand over the other when standing in the prayer, du‘aa’ following prayer,
Qunoot in Fajr prayer every day, and so on. 

So we should be more strict in objecting to the first type
(that concerning which there is scholarly consensus that it is an
innovation); objections to the second type (that concerning which the
scholars differed as to whether it is innovation) should be of a lesser
degree. In fact, one may refrain from objecting altogether and keep quiet
about it because there is such a difference of opinion concerning it. 

See the answer to question no.
70491 for information on
differentiating between issues concerning which the one who acts differently
should be denounced and those concerning which he should not be denounced. 

We should also note that Islam came to achieve and complete
that which is in people’s best interests and to ward off and reduce that
which is harmful to their best interests. 

In the event of a conflict between achieving something good
and warding off something bad, we should see which of the two is more
important and then give it precedence. 

Based on that, the Sunnah confirms that the hands should be
placed on the chest when standing in prayer, and the Sunnah is to recite
dhikr audibly following the obligatory prayer; each worshipper should recite
it audibly on his own, then if he offers supplication (du‘aa’) after
reciting this dhikr, he should do so quietly on his own. 

What they are calling you to do is not good and is contrary
to the Sunnah. 

But should you refuse to lead them in prayer so that you will
not end up doing something that is contrary to the Sunnah and committing
this wrong practice, or should you agree? 

The answer to that is as follows: 

If your refusing to lead them in prayer will result in their
bringing an imam who is more able than you to make these people follow the
Sunnah and teach it to them, and he will not end up going against the
Sunnah, then you should refuse. 

But if your refusal means that they will bring an ignorant
imam who will do these actions that are contrary to the Sunnah and add many
more to them, and he will not teach the people the Sunnah or adhere to it;
rather he may oppose the Sunnah and those who follow it, out of ignorance on
his part or because he is following his whims and desires, then in that case
you should not hesitate to agree to be an imam for these people, even if
they make you do these actions that are contrary to the Sunnah, because that
will lead to less negative consequences than if you refuse. 

We stated above that Islam came to ward off and reduce wrong
practices. Moreover, if you develop a close connection with them later on,
then you can try to teach them the Sunnah and lead them towards it little by
little. 

You can also tell them about the views of some scholars whom
they respect who spoke against what you want to denounce of innovations, so
long as you do that gradually so as not to alienate the people. 

There follow the views of some scholars about the imam
refraining from some things that he thinks are Sunnah and mustahabb, so as
to soften the hearts of the congregation. 

Shaykh al-Islam Ibn Taymiyah (may Allah have mercy on him)
said:

If the imam thinks that something is mustahabb and the
congregation does not think that it is mustahabb, and he refrains from doing
it for the sake of unity and harmony, then he has done something good. An
example of that is Witr, concerning which there are three scholarly
opinions, one of which is that it can only be three continuous rak‘ahs, like
Maghrib, as is the view of some of the Iraqi scholars. The second view is
that it can only be one rak‘ah that is prayed separately from whatever comes
before it, as is the view of some of the Hijazi scholars. The third view is
that both are permissible, as is the prevalent view of the Shaafa‘i madhhab,
Ahmad and others, and this is the correct view, however they favoured the
view that it should be separate from what comes before it. If the imam
thinks that it should be separate but the congregation favours the view that
Witr prayer should be done like Maghrib, and he goes along with that so as
to soften their hearts, then he has done well, as the Prophet (blessings and
peace of Allah be upon him) said to ‘Aa’ishah: “Were it not that your people
have only recently left Jaahiliyyah behind, I would have demolished (and
rebuilt) the Ka‘bah and made its door level with the ground, and I would
have given it two doors, one through which the people could enter and one
through which they could exit.” But he refrained from doing that which was
better in his view lest he alienate the people.

 Similarly, if a man thinks that the Basmalah (“Bismillah
ir-Rahmaan ir-Raheem”) should be recited out loud but he is leading in
prayer people who do not think that doing so is mustahabb, or vice versa,
and he goes along with them, then he has done well. End quote. 

Majmoo‘ al-Fataawa, 22/268 

Shaykh ‘Abd al-‘Azeez ibn Baaz (may Allah have mercy on him)
was asked: 

Is it permissible to refrain from saying Ameen out loud in
the prayer and not to raise the hands? 

He replied: 

Yes, if he is among people who do not raise their hands or do
not say Ameen out loud, then it is better not to do that, so as to soften
their hearts and be able to call them to what is good, and so that he can
teach them and guide them, and so that he will be able to teach them the
right way. That is because if he differs from them they will find that
objectionable, because they think that this is the Islamic way, and they
think that not raising the hands, apart from the opening takbeer, is the
Islamic way, and they have been used to that all their lives. The same
applies to not saying Ameen out loud, which is the subject of a well-known
difference of opinion among the scholars; some of them say that it should be
said out loud and some of them say that Ameen should not be said out loud.
According to the hadeeth, the Prophet (blessings and peace of Allah be upon
him) raised his voice, and according to other hadeeths he said it quietly.
Even though the correct view is that Ameen should be said out loud and it is
something mustahabb, and he will be omitting something mustahabb in this
case, the believer should not do something mustahabb if it will lead to
division, dispute and fitnah; rather the believer should refrain from doing
that which is mustahabb in that case. For the one who is calling people to
Allah, may He be glorified and exalted, if his refraining from something
will serve a greater interest, then he should do so. An example of that is
when the Prophet (blessings and peace of Allah be upon him) refrained from
demolishing the Ka‘bah and rebuilding it on the foundations of Ibraaheem. He
said: “Quraysh have only recently left kufr behind,” and it is for that
reason that he left it as it was and did not change it, in the public
interest. End quote. 

Fataawa Shaykh Ibn Baaz, 29/274,
275 

And Allah knows best.